docs: add contributing-plugins guide to docs-v0.56.x

[A-d-i-t-y]

What this PR does

Adds the missing "Contribute to Piped Plugins" section to the
docs-v0.56.x contribution guidelines page.

Which issue this fixes

Fixes #6786

Changes

• Added contributing-plugins/_index.md to
  docs/content/en/docs-v0.56.x/contribution-guidelines/
• Content includes: plugin architecture overview, plugin types,
  getting started guide, contributing to official and community plugins

Does this PR introduce a user-facing change?

Yes — users reading docs-v0.56.x can now find the plugin
contribution guide directly in the docs sidebar.

References

• Similar change done for docs-dev in PR docs: add contributing-plugins guide to docs-dev #6760

[A-d-i-t-y]

Hi @Warashi @khanhtc1202 @ffjlabo @t-kikuc @Ayushmore1214 @rahulshendre,

This PR adds the missing "Contribute to Piped Plugins" section to
docs-v0.56.x contribution guidelines. A similar change was done for
docs-dev in PR #6760.

Would appreciate a review when you get a chance!

[A-d-i-t-y]

Hi @khanhtc1202 @Warashi @rahulshendre @Ayushmore1214

Before this gets reviewed — wanted to check: is it appropriate to
add the plugin contribution guide to docs-v0.56.x, given that plugins
are a v1 feature? Happy to close this if it's not needed.

[A-d-i-t-y]

Hi @Warashi @khanhtc1202,

Based on the discussion in PR #6778, @rahulshendre has suggested
discussing the overall approach for docs-v0.56.x updates in the next
community meeting.

Should I close this PR for now and re-open after the meeting?

[A-d-i-t-y]

Closing this PR as it may not be appropriate for docs-v0.56.x
based on the discussion in PR #6778. Will wait for the community
meeting discussion before proceeding.