Improve docs project caching

This changes the caching method of our docs pipeline to make use of the new cache:key:files feature. The cache, which consists of the gems, and sometimes the gems and javascript files, will now be tied to changes to the Gemfile.lock and yarn.lock files, as needed.

The cache will only be rebuilt when those files change, so cache misses should be far rarer, and first pipeline runs will no longer need to generate all the caches. Instead, it will reuse the cache from previous runs within the project.

When the files change, the cache will be rebuilt a single time, then all subsequent jobs/pipeline runs will make use of the new cache automatically.