Update documentation: CI, general README.md, overview of doc README, various improvements

See:

• README.md
• doc/README.md
• doc/TESTBenches.md
• Other impacted files:
  • doc/RUNMODEM.md
  • doc/BUILD.md

What remains to be done:

• Align with wiki main page and delete there what is not relevant
• Add PLMN and frequency info for CI