Docs build process/systems

[rgcoe]

Description

This is an attempt to simplify and speed up the documentation pages build process.
To accomplish this it

• Uses sphinx-multiversion to generate documentation pages for latest and dev
• Add optional arguments for build_docs.py to skip running the example notebooks
• Fixes Docs: Theory animations not displaying #446
• Updates/improves instructions for building the docs in CONTRIBUTING.md

Type of PR

• Documentation

[coveralls]

Coverage Report for CI Build 27381980216

Warning

Build has drifted: This PR's base is out of sync with its target branch, so coverage data may include unrelated changes.
Quick fix: rebase this PR. Learn more →

Coverage remained the same at 87.938%

Details

• Coverage remained the same as the base build.
• Patch coverage: 5 of 5 lines across 1 file are fully covered (100%).
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	1426
Covered Lines:	1254
Line Coverage:	87.94%
Coverage Strength:	5.28 hits per line

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💛 - Coveralls

[jtgrasb]

@rgcoe I tested out the new build instructions. A few notes:

• Added a detail to contributing to make sure to use dev build before building docs locally.
• I got an error that source/_examples already exists, so I added an exception to copytree() if directory already exists.
• Added theory animations to the gitignore so it doesn't commit them when you build the docs locally.
  Merging now.