Made a start on documenting the code.

This seems less important for an application than a library - it's only useful to developers of Mastodonochrome - but still, it's practice using rustdoc, and also, I've been neglecting this project for a while and reminding myself of how it all works enough to document it is a good way to refresh my memory.

I've documented three modules in full: one self-contained library (coloured_string.rs), one moderately important subpart of the UI design (activity_stack.rs, including explaining what all the activity enum values mean), and one absolutely central part of the architecture (tui.rs).

enabled an automatic merge when the pipeline for 20c4daf4 succeeds

aborted the automatic merge because source branch was updated

added 4 commits

• 2daceb4d - Made a start on documenting the code.
• 976865b9 - Make Tui::run_inner into a closure.
• 3f78061b - Document some pending code cleanups.
• e5d0a595 - Make searching in files case-insensitive.

Compare with previous version

enabled an automatic merge when the pipeline for e5d0a595 succeeds

mentioned in commit 479d2b36

merged