Fixes #8179 - extraButtonLock and extraButtonPassword might be specified without a value or with 0 or with 1.

Refs #8204. Copy requiredPosition from Form to FormElement has been moved from 'formElementSetDefault()' to 'copyAttributesToFormElements()'

F7529 - Guarantee Sip Action plays only once. Update Doc, change from set to '' to removing SIP Key/Value completely.

Replace pdftk by pdfunite. 'pdftk' is outdated. It's hard to install on Ubuntu 18. It fails for recent PDFs. 'pdfunite' is based on poppler. URLs are preserved. Orientation and size are preserved.

Release.rst: fix typo
Sitemap.rst: new created to be included by Index.rst.

Reformat Index.rst according https://github.com/TYPO3-Documentation/TYPO3CMS-Example-ExtensionManual.git

Update ReST Syntax URLs. Remove 'extension/Documentation.outdated'. Update .gitignore to not commit rendered doc.