Update and restructure installation instructions

[vkucera]

• Simplify the structure of aliBuild installation instructions.
  • Reduce heading nesting depth from 7 to 4.
• Add instructions for aliBuild parallel jobs and for component-based build.
• Make a warning about keeping Git repositories up to date.
• Remove suggestion to remove sw as a clean up. Replace with a link to the actual cleanup.
• Unify the expressions in headings ("do" instead of "doing").
• Add links to the aliBuild homepage and alisw PPA.
• Move aliBuild intro before the aliBuild installation section.
• Move cleanup after installation use.
• Add a note about optional O2 source.
• Remove the redundant mention of best-effort support for Ubuntu.
• Remove unnecessary HTML tags.
• Simplify some sentences.
• Fix typos.

Ad restructuring:

For example, the actual aliBuild installation step for macOS is placed in a nonsensical and wrong hierarchy:

• Installing O2 and O2Physics
  • Installing aliBuild
    • Operating systems we support
      • Prerequisites
        • Platforms supported on a best-effort basis (all OS's are here)
          • aliBuild prerequisites for macOS
            • Install the required packages

This PR removes the 3 intermediate heading levels and puts it under:

• Installing O2 and O2Physics
  • Install aliBuild
    • macOS
      • Install the required packages