[docs] Add Mixed Precision detailed docs

[SeanNaren]

What does this PR do?

Re-introduces a mixed precision set of docs. This is primarily to explain bfloat16, and give an overview of the various options. There are some duplication of docs however I think this is fine primarily for discoverability.

We also need a reference point for bfloat16 to share :)

Before submitting

• Was this discussed/approved via a GitHub issue? (not for typos and docs)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests? (not for typos and docs)
• Did you verify new and existing tests pass locally with your changes?
• Did you list all the breaking changes introduced by this pull request?
• Did you update the CHANGELOG? (not for typos, docs, test updates, or internal minor changes/refactorings)

PR review

Anyone in the community is welcome to review the PR.
Before you start reviewing make sure you have read Review guidelines. In short, see the following bullet-list:

• Is this pull request ready for review? (if not, please submit in draft mode)
• Check that all items from Before submitting are resolved
• Make sure the title is self-explanatory and the description concisely explains the PR
• Add labels and milestones (and optionally projects) to the PR so it can be classified

Did you have fun?

Make sure you had fun coding 🙃

[tchaton]

LGMT !

[codecov]

Codecov Report

Merging #9104 (1423b3c) into master (bac8b1b) will increase coverage by 4%.
The diff coverage is n/a.

❗ Current head 1423b3c differs from pull request most recent head e2e776a. Consider uploading reports for the commit e2e776a to get more accurate results

@@           Coverage Diff           @@
##           master   #9104    +/-   ##
=======================================
+ Coverage      88%     92%    +4%     
=======================================
  Files         176     176            
  Lines       14666   14666            
=======================================
+ Hits        12889   13500   +611     
+ Misses       1777    1166   -611     

[mergify]

@codecov[bot] is not allowed to run commands

Hey, I reacted but my real name is @Mergifyio

[ethanwharris]

LGTM 😃