Doc: deprecate Cbuild documentation

[vovamarch]

Legacy CBuild documentation is still available at https://open-cmsis-pack.github.io/devtools/buildmgr/latest/index.html . It is the only place with CPRJ description, so probably needs to be kept alive. But non-relevant items can be removed and a deprecation note plus redirection to CMSIS-Tooblox docs can be added.

[jkrech]

align with deprecation of cbuildgen

[ReinhardKeil]

Only the Open-CMSIS-Pack spec is open: remove CMSIS-Build from this documentation

[brondani]

I would suggest to remove the documentation at the same time of removing the code and workflows related to CMSIS-Build.

Proposal of an action plan:

• Update toolbox release workflow (complementary to Update the toolbox release flow to directly collect binaries cmsis-toolbox#135)
• Move cmake config files to projmgr or to cbuild2cmake. In alternative implement [cbuild2cmake] Create toolchain configuration yml files to replace cmake files cbuild2cmake#123.
• Remove buildmgr documentation and source code base.
• Remove buildmgr build, test, release and documentation workflows.
• Remove cbuildgen and cprj related features from projmgr and cbuild.