Documentation for performance is fragmented

The documentation about performance is in different places:
* https://docs.bazel.build/versions/master/guide.html#profiling
* https://docs.bazel.build/versions/master/skylark/performance.html (some of it doesn't seem specific to Starlark)
* https://docs.bazel.build/versions/master/memory-saving-mode.html

I think it would be useful to consolidate the information in one place.

This blog post may also contain information to put in the documentation: https://filipnikolovski.com/posts/bazel-performance-in-a-ci-environment

Start by comparing the profiling guide, skylark/performance.html, and memory-saving-mode.html, then review the linked CI performance blog post for relevant material. Consolidate the performance information into one documented place, preserving the useful content and making the resulting documentation easy to find.

Written by the indexing model from the issue text.