[DOCS] Align with new TYPO3 documentation standards

- align README.rst, Index.rst, Includes.txt, Includes.rst.txt,
  Settings.cfg
- add genindex.rst, Sitemap.rst
- remove outdated Targets.rst
- rename Includes.txt to Includes.rst.txt
- reference manual's start page with :doc:`<manual>:Index`

Adding the custom label `start` to the beginning of the manual's
Index.rst is redundant. Use :doc:`<manual>:Index` instead of
:ref:`<manual:start>` to refer to it.

- replace :ts: with :typoscript: text role

The ambiguous :ts: text role has been removed to
not confuse the writer with typescript and typoscript.

- fix rendering warnings of EXT:core
- fix rendering warnings of EXT:dashboard
- fix rendering warnings of EXT:fluid_styled_content
- fix rendering warnings of EXT:form
- fix rendering warnings of EXT:linkvalidator

- remove outdated encoding note
- align reST validator at Build/Scripts/validateRstFiles.php

See https://docs.typo3.org/m/typo3/docs-how-to-document/main/en-us/GeneralConventions/FileStructure.ht...