Support comments when generating documentation

[Vrixyz]

• This builds on [naga] Comments parsing (naga + wgsl) gfx-rs/wgpu#6364 and Add support for comments bevyengine/naga_oil#108 to display comments.

This PR also adds support to target documentation on a local path. It copies the whole directory and parses that. But it has a few limitations:

• I imagine we could parse directly the provided path, but I went the "easy way"
• some links are hardcoded to crates.io, so we'd need to improve that to target local dependencies (maybe compile said crate locally too? and/or provide hybrid solution?)

I added a small diagram I did to understand the architecture, let me know if it's welcome or should I remove it.

[Vrixyz]

Version is a bit useless when targeting local, but we'd have to rework the dropdown choice to support local.

[jannik4]

Hey, I would like to update to support bevy 0.15. Since you already did this here, would you like to split this PR and we can merge the changes for 0.15? If not, I would just make a separate PR and you would have to rebase when the upstream changes are ready.

[Vrixyz]

I believe my work on this was before the whole naga update in bevy, but I'll open a split PR to get a discussion started at #2 :)