[TASK] Deprecate DocumentTemplate methods

This patch deprecates the following parameters/methods within
DocumentTemplate:

* sectionFlag (is internal)
* viewPageIcon()
* getHeader()
* getResourceHeader()
* header()
* icons()
* t3Button()
* wrapScriptTags()
* loadJavascriptLib()
* getContextMenuCode()

Resolves: #72859
Releases: master
Change-Id: I589c364ec506793439ac007b002cf28a6e1ca69d
Reviewed-on: https://review.typo3.org/47769


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>