Add documentation link to extension information

Final goal: Render the link to docs in the extension manager's list. To achieve this, first the extension.xml must be enhanced with a new field "documentation" which holds the URL just as e.g. in https://extensions.typo3.org/extension/news/

IMO this must be done on extensions.typo3.org as there already the fallback chain of Documentation/ > Readme.rst > Readme.md > external link > is known and rendered.

Once this is achieved we can improve the core

AC

• Add a new property documentation_link to extension.xml which points to external documentation link or to the detail page of the extension