Add technique for offline generation of autocompletion files to docs

[morags]

Summary

Generating uv and uvx's autocompletions can take over half a second on even a moderately fast system, while loading pre-prepared files can be two orders of magnitude faster. This commit provides users with a general-purpose technique for generating autocompletion files offline, which could contribute to significantly faster shell startup times.

Test Plan

Documentation change - manual testing

[zanieb]

Hm it only takes 5ms on my machine, you're seeing half a second? That seems way too slow.

Separately, this might be nice to have but doesn't seem appropriate for our installation guide — it's too detailed. We'll want to find somewhere else for it, e.g., in the Reference, but I'm not sure where would be best yet.

[morags]

~300ms after some upgrades, yes. Then the cache kicks in and subsequent runs are much faster, but obviously only the first one matters.

I know it's general-purpose, but uv is the main culprit in my scenario, hence the uv docs. In the meanwhile I put it in its own gist, in case it doesn't fit here.