[CLEANUP] PageTS TCEmain page

Cleaned up the page about TCEmain options is Page TSconfig.
Added section headings for each property, added local TOC,
renamed images, etc.
[FEATURE] Clear cache by tag

Document the possibility to use tags as values for
clearCacheCmd.

Releases: 6.1, 6.2
[BUGFIX] Correct wrong example

Correct wrong example in newly introduced code sample.

Releases: 6.1, 6.2
[TASK] Update Feedback section

As discussed in the ML, the Feedback section at the start of all
official manuals needs to be updated to mention GitHub.
[TASK] Open version 7.0

Set manual version to 7.0-dev.