Update installation instructions for easier navigation on icinga.com

[bobapple]

This PR:

• Splits installation documentation into separate Operating System navigation items for icinga.com
  • Splitting into separate files is achieved by using conditions within 02-installation.md. Based on the conditions the script parse_templates.py from https://github.com/icinga/icinga-docs-tools separates the installation instructions into files during the generation of the docs for icinga.com
• Introduces new supported systems like Amazon Linux 2
• Updates installation instructions, especially for RHEL, SLES and AL2
• Cleans up some old paragraphs
• Re-arranges the different sections to make the installation process more clear

[bobapple]

@cla-bot check

[julianbrost]

@bobapple Will you ensure that these changes are also forward-ported to the master branch?

[julianbrost]

The changes already received some rounds of internal review, so the current version is fine for me.

Changes still have to be forward ported to master but there are some conflicts that one has to manually look into and resolve for which I don't have time right now, so I created #9218 for now so that it won't be forgotten.