Migrate docs to php-based rendering

[rintisch]

Feature Request

Is your feature request related to a problem? Please describe. The new doc does no longer use Sphinx but phpDocumentor/guides, see https://docs.typo3.org/other/t3docs/render-guides/main/en-us/Introduction/Index.html which has better A11y.

Describe the solution you'd like Migrate the documentation to use phpDocument/guides: https://docs.typo3.org/other/t3docs/render-guides/main/en-us/Migration/Index.html I would prefer to wait until the search is properly reimplemented which is not achieved at the moment of writing this ticket.