add preview docs section

[epassaro]

Description

Add a documentation section on how to build docs in your fork to preview changes (and check you are not breaking the docs!).

Motivation and Context

Sometimes it's useful to share a preview of changes in documentation. But the most important thing is this will help to make the pipeline break less often (if users are encouraged to preview changes in documentation before merging).

How Has This Been Tested?

See: https://github.com/epassaro/tardis/actions/
and https://epassaro.github.io/tardis/development/documentation_preview.html

Screenshots (if appropriate):

Types of changes

• 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 not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have assigned/requested two reviewers for this pull request.

[codecov]

Codecov Report

Merging #1383 (b7aebef) into master (23ac153) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1383   +/-   ##
=======================================
  Coverage   72.38%   72.38%           
=======================================
  Files          66       66           
  Lines        5098     5098           
=======================================
  Hits         3690     3690           
  Misses       1408     1408           

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