Full review/update of guide, adding new section

This is a pretty major revision.

• Lede was originally too vague to be informative.
• TOC uses markdown links instead of html.
• Headers no longer use <a name>, use the auto-generated links instead.
• New section provides basic information about all the sub-projects and how issues are moved between them. Targets new contributors.
• New tasks: milestones, separating issues, confidential issues.
• New tips: search, running multiple instances
• PlantUML diagram is simplified to act as an overview. Too much detail isn't very informative and makes the diagram harder to read. Left out task like handling support requests, and closing duplicate reports.
• Add more detail to feature requests, UX issues, and labelling issues.
• Add more detail to checking preferences and collecting backtraces.
• inbox no longer uses light labelling.
• Removal of a few rules/exceptions that I don't think apply anymore
• Various changes to wording and heavy restructuring of whole sections. Preference descriptions/reasoning over short instructions.

I considered using an auto-generated table of contents ([[_TOC_]]) but preferred the numbering from the manual TOC.

Current revision from this MR can be viewed here: https://gitlab.com/inkscape/inbox/-/blob/0ff6a8b47d1fbfba120f892c59deb2a47fea191f/Guidelines.md
Same as above but commits squashed: https://gitlab.com/inkscape/inbox/-/blob/7d7c57012540a57a1a88577bba8f7e9de3a2bb4b/Guidelines.md
Version previously in inbox: https://gitlab.com/inkscape/inbox/-/blob/5d6ffba9033f4b3e3da34006b88a8c87335fa174/Guidelines.md