Reorganise table of contents

[itowlson]

This attempts to address the issues noted in #4 (comment).

• I've reorganised the applications section to writing->structure->building->running (line) testing->observing->troubleshooting (line) config
• I've made triggers their own section, and each trigger is now top-level. The cron trigger entry is a bit weird @flynnduism, it seems to cite the /redis-trigger and /triggers page - what was your intent here?
• I've reorganised the API guides section and reverted the name changes (e.g. I have a very strong preference for "Making HTTP Requests" over "HTTP Interface").
• Moved the reference to a reference section
• Moved a bunch of internals and nerd stuff into a "project" section. I don't love this name but found things like data layout and contributing guides and plugin authoring stuff without a good home... it's all kind of stuff for people who are developing Spin-related stuff rather than building Spin apps. Better ideas welcome
• Moved user-facing plugins and templates stuff into a Tools group (again not the ideal name but they definitely didn't belong in the applications group). Restored the "managing" name to distinguish them from authoring.

I am not sure if I've mucked up the styles or something, but locally it now looks like this for me:

which seems a lot more spread out (and the section headers a lot smaller) than I remember in the preview from #4 - but I may be misremembering!

[vdice]

@itowlson if you'd like, a PR deployment preview will be available if you rebase on main -- but you'd also have to re-create the PR on origin for it to work.

[itowlson]

@vdice I've compared how it looks vs how other previews look and concluded that it was my memory that was messed up rather than my PR.