add cache backend docs (upstream)

[dvdksn]

Add stubs and config for fetching cache backend docs from upstream repo.

also fixed minor issue in the cache introduction doc (introduced by myself earlier)

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[jedevc]

LGTM to me once rebased 😄

htmlproofer not happy

[crazy-max]

need to fix some links upstream: https://github.com/docker/docs/actions/runs/3225994881/jobs/5278977353#step:5:57

looks like we are using relative links:

- ./_site/build/building/cache-backends/index.html
  *  linking to internal hash #cache-from that does not exist (line 172)
     <a href="../../reference/buildx_build.md#cache-from"></a>
  *  linking to internal hash #cache-to that does not exist (line 170)
     <a href="../../reference/buildx_build.md#cache-to"></a>
  *  linking to internal hash #secret that does not exist (line 128)
     <a href="../../reference/buildx_build.md#secret">--secret</a>

[dvdksn]

Seems like changing from /cache-backends to /cache/backends broke some more relative links. Raised a PR to update the links in upstream: docker/buildx#1353