docs: RST double-backtick notation breaks API cross-reference links

[planetf1]

Problem

Several API doc pages display cross-references as raw markdown text instead of clickable links.

Example — mellea.backends.backend:

FormatterBackend extends the abstract ``Backend`` with a ``ChatFormatter``…

The backtick-wrapped link text is rendered as inline code containing raw link syntax rather than a rendered link.

Root cause

91 source files use RST-style ````Symbol```` double-backtick notation in docstrings (Sphinx convention). The `add_cross_references` build step matches the inner single-backtick boundary and generates a link wrapped in an extra code span:

```
Backend → [Backend](url) ← Mintlify renders as code, not a link
```

Fix (this PR)

• decorate_api_mdx.py: new normalize_rst_backticks() pass converts ``x`` → `x` in MDX prose before cross-reference substitution runs
• validate.py: new validate_rst_docstrings(source_dir) warns on RST notation without blocking the build

Validation output (CI)

Running `uv run poe apidocs-validate` with `--source-dir mellea` now reports:

✅ RST docstrings: FAIL
   992 occurrences found

Overall build remains green — this is a warning only.

Follow-up

See #657 — convert ``Symbol`` → `Symbol` across source files (91 files, 992 occurrences).

[planetf1]

Before 'fix'

After fix