Initial draft architecture for FAQ/Troubleshooting.

[bobbyxng]

Changes proposed in this Pull Request

• Following @nworbmot's suggestion, proposing a draft skeleton for a potential FAQ/troubleshooting guidelines for common model issues/challenges
• Open for discussion and FAQ proposals
• https://pypsa-eur--1939.org.readthedocs.build/en/1939/

Checklist

• I tested my contribution locally and it works as intended.
• Code and workflow changes are sufficiently documented.
• A release note doc/release_notes.rst is added.

[fneum]

Sure, great idea! But would keep it simple without the expansion: better Ctrl+F experience and less code.

[bobbyxng]

Exchange @toniseibold @bobbyxng on structure, inspired by open-energy-transition#276:

• General
• Solving
• Model customisation
  (Constraints, input data, preprocessing...)
• Results & postprocessing
• Contribution & support

[bobbyxng]

@toniseibold and I added an initial set of questions, answers and also transferred some boxes from our OET colleagues (open-energy-transition#276). We added some sections for a basic structure and removed the expanding boxes following your comment to keep it simple, but I do think the admonition style boxes are quite nice, visually. @fneum: Could you have another look?