Docs: Add new section "How-to / Troubleshooting" and move 2 existing troubleshooting pages

[benjaoming]

Refs: #9746

This is a fairly simple change:

• Create a new sublevel in navigation: "How-to guides / Troubleshooting"
• Move "Troubleshooting build errors"
• Move "Troubleshooting slow builds"
• Adds a new type of suggestion to send in comments and ideas (because Troubleshooting can benefit from the additional grounding in direct user feedback)

I imagine that we will be able to write troubleshooting guides much quicker in the future, given that they have an easily-expandable section now.

Some FAQ items might go and live there in the future.

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📚 Documentation previews 📚

• User's documentation (docs): https://docs--9914.org.readthedocs.build/en/9914/

• Developer's documentation (dev): https://dev--9914.org.readthedocs.build/en/9914/

[benjaoming]

I just realized some other Troubleshooting content to be relabled - so launching "How-to guides / Troubleshooting" is already on the table and easier done in one PR 👍

[ericholscher]

Lots of comments for a tiny PR, but I think a new guide section is worth thinking about! Exciting.