feat(live): Improve Makefile for building the Live ISO

[lslezak]

Improvements

• Allow building the SLE image (using internal OBS)
• Allow passing extra parameters to OSC build (like preferred RPMs)
• Updated documentation
  • Documented the new features
  • Removed outdated Cockpit section
  • Unified heading style

[coveralls]

Pull Request Test Coverage Report for Build 13200012423

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 5 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.02%) to 70.888%

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Files with Coverage Reduction	New Missed Lines	%
rust/agama-server/tests/common/mod.rs	5	74.19%

Totals
Change from base Build 13198839713:	-0.02%
Covered Lines:	17291
Relevant Lines:	24392

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💛 - Coveralls