[TASK] Re-order Frontend Content building process

This patch streamlines the execution code when a base page is generated
by moving all logic into a few smaller methods to separate the actual
"cacheable page" logic from the PageRenderer population process,
and also adds more documentation to the methods.

In addition, some minor adjustments (site language is now
always available in v10) can be made, as some unnecessary
code execution (e.g. divSection assigment and unsetting
"JScode" explicitly) are made

Resolves: #89663
Releases: master
Change-Id: Ic2ac15a9767f452a89bace0d95fdc6f7f09af922
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/62313


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Susanne Moog <look@susi.dev>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Susanne Moog <look@susi.dev>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>