Update docs for overriding rapids-cmake

[KyleFromNVIDIA]

Description

#257 updated the readme, but forgot to update basics.rst. Copy the text from the readme.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.
• The cmake-format.json is up to date with these changes.
• I have added new files under rapids-cmake/
  • I have added include guards (include_guard(GLOBAL))
  • I have added the associated docs/ rst file and update the api.rst

[jameslamb]

This is great, thanks!

[robertmaynard]

/merge