docs: Improve demo and troubleshooting sections

[KonradBkd]

Changes

• Extended build instructions for demo, troubleshooting section
• Changed demo and utilitiy docs to rst description lists (instead of rst tables)
• Fixed various paragraphs and typos
• Centralized substitutions
• Restructured file structure to represent doc structure
• css for doxygenfunction

Review Instructions

• Build the Docs
• Carefully read the sections with major changes:
  • Demos
  • Troubleshooting
  • API (just the overview page)
• Review the root structure. Changed chapter titles:
  • 'Simulation' -> 'Simulation Concetps'
  • 'API' -> 'C/C++ API'
• Condensed 'SIL Kit License' AND 'Third Party Licenses' on the root structure to just 'Licenses'
• Check the API sections that use breathe directives (doxygenfunction, doxygenclass, ...).
  The custom.css has selectors that effect all of them.

[VLukasBecker]

I would suggest to increase the nesting level of the content list of the docu, inorder to make the Vehicle Network Services selectable from the list, as it was possible before.

[VLukasBecker]

The rest lgtm!

[KonradBkd]

I would suggest to increase the nesting level of the content list of the docu, inorder to make the Vehicle Network Services selectable from the list, as it was possible before.

I increased the global navigation_depth from 4 to 5, no it reaches the Vehicle Network Services again

[VLukasBecker]

lgtm