Build performance optimizations for projects with large sidebars

[HiDeoo]

Description

This PR is a draft experimenting with a new approach to fix #2215 by caching generated sidebars per locale to avoid regenerating them for each page during the build process.

A few details about the approach:

• It's using a basic Map at the moment.
  • I still want to try this approach against the SSR PR and maybe test with an LRU instead.
• The approach is used for the docs content collections pages and <StarlightPage> not specifying a custom sidebar (which means the sidebar generated based on the global sidebar config is used).
  • Using the same approach for <StarlightPage> with a custom sidebar would probably require a hash of the custom sidebar config so I decided to keep it simple for now and it's not required to get an idea of the potential performance improvements (there is also no guarantee that generating such a hash would not kill the performance improvements from the caching).

Here are some benchmarks I ran on my machine:

Starlight Docs

• Images are cached from a previous run.

Before:

❯ hyperfine --runs 5 'pnpm build'
Benchmark 1: pnpm build
  Time (mean ± σ):     23.290 s ±  0.592 s    [User: 34.546 s, System: 3.046 s]
  Range (min … max):   22.431 s … 23.840 s    5 runs

After:

❯ hyperfine --runs 5 'pnpm build'
Benchmark 1: pnpm build
  Time (mean ± σ):     22.000 s ±  0.886 s    [User: 32.500 s, System: 2.769 s]
  Range (min … max):   21.124 s … 23.097 s    5 runs

Cloudflare Docs (no patch)

• Images are cached from a previous run.
• Default Starlight with the changes from this PR applied without the Starlight patch that is applied to the Cloudflare docs.
• astro check has been removed from the build npm script as it's not really relevant for this benchmark.

Before:

❯ hyperfine --runs 5 'npm run build'
Benchmark 1: npm run build
  Time (mean ± σ):     363.060 s ±  2.154 s    [User: 394.678 s, System: 24.588 s]
  Range (min … max):   360.170 s … 366.090 s    5 runs

After:

❯ hyperfine --runs 5 'npm run build'
Benchmark 1: npm run build
  Time (mean ± σ):     230.533 s ±  2.276 s    [User: 264.999 s, System: 22.543 s]
  Range (min … max):   228.171 s … 233.879 s    5 runs

Cloudflare Docs (with patch)

• Images are cached from a previous run.
• Starlight using the changes from this PR and an updated version of the Starlight patch that is applied to the Cloudflare docs (updated so that the cache of intermediate sidebars is keyed by locale + current section instead of just locale).
• astro check has been removed from the build npm script as it's not really relevant for this benchmark.

Before:

❯ hyperfine --runs 5 'npm run build'
Benchmark 1: npm run build
  Time (mean ± σ):     210.619 s ±  2.512 s    [User: 240.743 s, System: 22.913 s]
  Range (min … max):   208.476 s … 214.525 s    5 runs

After:

❯ hyperfine --runs 5 'npm run build'
Benchmark 1: npm run build
  Time (mean ± σ):     200.607 s ±  3.295 s    [User: 229.740 s, System: 21.582 s]
  Range (min … max):   196.067 s … 204.917 s    5 runs

A few interesting points:

• For the Starlight Docs, the improvement is not really significant but it's expected as it's mostly targeting sites with large sidebars.
• For the Cloudflare Docs with their current patch updated, the improvement is also not that huge but I think it's also expected as their current patch already trimmed down the number of entries generated for the sidebars a lot.
• For the Cloudflare Docs without the patch, the improvement is a bit more interesting, especially considering that most people will not have similar use cases and optimizations in a custom patch.

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[HiDeoo]

Undrafting the PR as I've been playing quite a bit with the changes and didn't see any downside to the current approach. I still want to do a little bit of play-testing with some SSR scenarios but I thought I might as well undraft the PR before I forget as usual and in case some users want to provide some feedback in the meantime.