Document rationale for vendoring WebGPU bindings.

[jimblandy]

This commit is almost entirely autogenerated changes.

[Wumpf]

nice writedown, thank you!

[daxpedda]

You may want to mention that this also serves as a workaround for --cfg=web_sys_unstable_apis, which would be required when depending on unstable APIs in web-sys.

[jimblandy]

It turns out that #5751 manually patched the vendored files, so my re-vendoring broke uses of GpuFeatureName::TextureCompressionBcSliced3d. I'm going to patch it back in manually to let this PR land, and then file a follow-up issue to remove it.

@mehmetoguzderin @teoxoy We can't generally have local changes to the files in wgpu/src/backend/webgpu/webgpu_sys, unfortunately.