Documentation updates and new SlurmStopping callback

[alphataubio]

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Motivation and Context / Related issue

• Documentation updates ( PREVIEW https://alphataubio.com/nevergrad/ )

  • sphinx docs refactored by sections (contents, api reference, examples, statistics)
  • automatic venv in docs/Makefile
  • Callbacks API split into its own ref page
  • minor editing to many pages
  • new "Strict constraints in continuous optimization" page

• new SlurmStopping callback (EarlyStopping when 5 minutes left in SLURM job, no-op if not in SLURM job)

How Has This Been Tested (if it applies)

github action

Checklist

• The documentation is up-to-date with the changes I made.
• I have read the CONTRIBUTING document and completed the CLA (see CLA).
• All tests passed, and additional code has been covered with new tests.

[alphataubio]

@teytaud "This workflow is awaiting approval from a maintainer in #1692" for the tests to run on commit

[alphataubio]

@teytaud PR1692 ready for review and merge