Fix broken internal links

[Watson1978]

Summary

Audited all 206 markdown files (2,774 links) plus the 177 redirects in .gitbook.yaml, and fixed every internal link that does not resolve — 63 links across 14 files, 3 redirects, and one new file.

External URLs are deliberately out of scope here; they need per-URL investigation for replacements and will be sent as a separate PR.

Path fixes (56)

Where	Problem	Fix
installation/install-calyptia-fluentd/*.md (51 links)	These four install guides live one directory deeper than the links assume, so every relative link is short by one level.	Add one ../. This matches the sibling install-fluent-package/ guides, which already use ../../configuration/config-file.md and ../before-install.md.
installation/install-fluent-package/install-by-dmg-fluent-package.md (1)	Same problem on the link to install-by-gem.md; the other guides in that directory already use ../install-by-gem.md.	../install-by-gem.md
installation/install-by-{deb,rpm}-td-agent-v4.md (2)	Points at ../quickstart/td-agent-v2-vs-v3-vs-v4.md, but that page is in appendix/.	../appendix/td-agent-v2-vs-v3-vs-v4.md
how-to-guides/apache-to-s3.md (1)	../output/s3 is missing the .md extension.	../output/s3.md
how-to-guides/http-to-td.md (1)	../installation/post-installation-guide#plugin-management is missing the .md extension.	../installation/post-installation-guide.md#plugin-management

New file (1)

SUMMARY.md lists * [Appendix](appendix/README.md), but that file does not exist, so the Appendix section has no landing page. Added appendix/README.md with a short introduction and links to the two articles it contains.

Anchor fixes (7)

These were verified against the id attributes actually rendered on docs.fluentd.org, not against a local build — honkit generates heading anchors by different rules than GitBook does, so a local build cannot be used to check them.

Where	Old	New
deployment/system-config.md	source-only-mode.md#Recovery	source-only-mode.md#recovery
output/forward.md (2 links)	../input/forward.md#how-to-enable-tls/ssl-encryption	../input/forward.md#how-to-enable-tls-encryption
configuration/config-file.md	config-file.md#5.-group-filter-and-output-the-label-directive	config-file.md#id-5.-group-filter-and-output-the-label-directive
filter/README.md	../plugin-development/#filter-plugins	../plugin-development/api-plugin-filter.md
formatter/README.md	../plugin-development/#text-formatter-plugins	../plugin-development/api-plugin-formatter.md

The heading in input/forward.md is now "How to Enable TLS Encryption", so the old tls/ssl anchor no longer exists. The "Filter Plugins" and "Text Formatter Plugins" sections were removed from plugin-development/README.md and are now separate pages, so those two links point at the pages instead of at anchors that no longer exist.

GitBook prefixes id- to anchors generated from headings that start with a digit, so the id of "5. Group filter and output: the label directive" is id-5.-group-filter-and-output-the-label-directive. GitBook's own on-page table of contents links to it that way, but the markdown link in the body did not. It is the only link in the repository affected.

Redirect fixes in .gitbook.yaml (3)

Most redirect targets name paths that no longer exist in the repository, but GitBook tracks those file moves internally and resolves them anyway — for example v1.0/articles/in_http still points at plugins/input/http.md and correctly lands on /input/http. I checked all 177 against the live site; three of them return 404, because those pages were never at the recorded path for GitBook to track (plugins/output/opensearch.md and developer/api-plugin-helper-metrics.md have no history at all, and plugins/formatter/tsv.md was deleted and re-added, which broke the chain).

Redirect	Old target	New target
v1.0/articles/out_opensearch	plugins/output/opensearch.md	output/opensearch.md
v1.0/articles/formatter_tsv	plugins/formatter/tsv.md	formatter/tsv.md
v1.0/articles/api-plugin-helper-metrics	developer/api-plugin-helper-metrics.md	plugin-helper-overview/api-plugin-helper-metrics.md

The remaining 174 redirects resolve correctly and are left untouched.

Verification

Three independent checks:

1. Source analysis — resolve every link target against the working tree (code blocks and inline code spans excluded). 58 unresolved before, 1 after (see below).
2. honkit build crawl — honkit build and walk every href/src in the output. 47,503 relative links, 0 unresolved.
3. Live-site check — fetch each target page from docs.fluentd.org and compare against the real id values, and request every redirect source. 110 anchor-bearing links, 7 broken before, 0 after; 177 redirects, 4 broken before, 1 after (see below).

codespell passes.

Deliberately left for a follow-up PR

plugins/input/dummy.md is the only file left under plugins/. It is not listed in SUMMARY.md, so GitBook does not publish it, which is why v1.0/articles/in_dummy returns 404 — and its own link to /input/sample.md is a root-absolute path that does not resolve. in_dummy was renamed in_sample in v1.11.12 but the plugin still ships, so the page should move to input/dummy.md and be added to the TOC rather than be dropped. That is a file move plus a SUMMARY.md change, so it is kept out of this PR to avoid conflicts.

Also out of scope

• 12 external URLs (21 occurrences) are broken or redirected. Separate PR.
• 5 http://localhost:* links are operational examples inside walkthroughs, not navigation.

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