Add inventory_cache arg to sphinx_runner

[phlax]

Currently the sphinx_runner reads a config file in the Envoy/website repos and downloads the inventories on every docs build

It would be useful to add an inventory_cache arg that read from a directory and/or tarball to find the inventory files

this would allow bazel some control over how the inventories were fetched/found

in the case of the website the inventory files are present in the repo/workspace so they can be used directly

for Envoy, probably the best plan is to add them directly to the repo as they rarely change, but the challenge here is that we would then need CI to ensure this was up to date

one option would be to add a --strict-inventory-cache or similar that made sure that all/only the expected inventory files were present

[phlax]

Seems like this is not what is required. Sphinx config allows you to specify (multiple) sources for the objects.inv file so we can just specify and bundle the inventory files

What we do need is something to download (and maybe sync) the inv files

[phlax]

closed in favour of #462