Ad-hoc: Rustdoc frontend benchmarking

[jyn514]

Currently, rustdoc has perf tests for the CLI runs, but not for how long it takes to load a given page. @GuillaumeGomez tells me we do have tests for search results loading (yay!) but we still don't have tests for anything else.

It would be nice if we could benchmark load times, maybe using something like https://developer.chrome.com/docs/lighthouse. That would also help us look at prospective new features to the frontend and make decisions based on data rather than having to worry about how it works on mobile browsers.

[jyn514]

GuillaumeGomez tells me we do have tests for search results loading (yay!) but we still don't have tests for anything else.

I think I may have misunderstood this — we have tests for search, but not benchmarks.

@yotamofek points out that just benchmarking the JS perf, not the whole HTML loading, would be quite simple to setup and we could get it working pretty quickly with Node.

Ideally, this would hook into x bench, or into the runtime benchmarks (I think @Kobzol has been working on something like that?) but that can be a follow-up, we can start with something standalone.

[Kobzol]

I don't think that this is a good fit for rustc-perf, tbh, it would be too different from the way the existing benchmarks are set up and would probably require a lot of changes. Adding it to x bench probably shouldn't be a problem though.