Automatic index and cross-reference support.

[awstanley]

Description:

• Added custom Sphinx extension (esphome-sphinx.py) and enabled it in conf.py.
• All component and device pages modified to add metadata.
• Replaced manually handled tables in index.rst with automatically generated tables.

The changes mean that individual rst files may be moved around directories and the extension will automatically append them to the table.

Basic cross-referencing support added to device pages to prove compliance/support.

Checklist:

• [x] Branch: next is for changes and new documentation that will go public with the next ESPHome release. Fixes, changes and adjustments for the current release should be created against current.
• ~~Link added in /index.rst when creating new documents for new components or cookbook.~~ Not applicable.

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[awstanley]

Difficult to diagnose (and apparently impossible to replicate?) multiprocessing/parallel Sphinx issue makes this less than viable at this time.

[awstanley]

Resolved against multiple hosts with the internal error (hopefully) diagnosed.

[github-actions[bot]]

There hasn't been any activity on this pull request recently. This pull request has been automatically marked as stale because of that and will be closed if no further activity occurs within 7 days. Thank you for your contributions.