Update user guide

[jemrobinson]

✅ Checklist

• You have given your pull request a meaningful title (e.g. Enable foobar integration rather than 515 foobar).
• You are targeting the appropriate branch. If you're not certain which one this is, it should be develop.
• Your branch is up-to-date with the target branch (it probably was when you started, but it may have changed since then).

🚦 Depends on

n/a

⤴️ Summary

Full rewrite of the user guide to reflect changes made as part of v5 migration.

I've tried to use:

• user: ada.lovelace
• SHM: projects
• SRE: sandbox
• domain: projects.example.org

throughout, but let me know if I've missed anywhere.

Link to the preview build on readthedocs

🌂 Related issues

n/a

🔬 Tests

Documentation only

[github-actions]

Coverage report

This PR does not seem to contain any modification to coverable code.

[JimMadge]

Looks good. I've suggested a few changes.

I think getting the balance between what goes here and what belongs in another kind of documentation is difficult,

• Instance specific stuff like policies, processes
• How much to document about the tools we provide.