Baseline: 2f9c7d60ea676f9f8e929b20a3ff74e786995c91

Cherry picks:

   + 9d92e7189bd7996286cbc0b04ae5c3ef7894f23c:
     Small tweaks to platform documentation. - Point platform rules
     to the explanatory document which people should have read first.
     - Update the left nav to include the word "Toolchains" - move
     https://bazel.build/extending/platforms from Design Docs to
     Concepts
   + d5095dbfd320654298aaa1b412a7581b53792d3f:
     "bazel config" output tests: skip noconfig.
   + 923425002daae6f83cb0c6487cc3453b2dc89a43:
     Automated rollback of commit
     0fe4c36a7feb24444bc72731bf4839aa1ca926c6.

Incompatible changes:

  - --experimental_build_transitive_python_runfiles is flipped to
    false. See #16303 for details

Important changes:

  - Now that the host configuration is finished, `genrule` should
    prefer the use of `tools` and stop using `exec_tools`.

This release contains contributions from many people at Google, as well as Fabian Meumertzheim, Keith Smiley.