Add custom widgets documentation

[PardhavMaradani]

Changes made in this Pull Request:

• Added documentation for custom widgets

PR Checklist

• Tests?
• Docs?
• CHANGELOG updated?
• Issue raised/referenced?

---

Hi @orbeckst, @jeremyleung521, @amruthesht, @HeydenLabASU,

This PR adds documentation for adding custom widgets.

Here is the RTD preview link: https://mdadash--66.org.readthedocs.build/en/66/

Thanks

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (b885886) to head (a21b19b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

Components	Coverage Δ
frontend	100.00% <ø> (ø)
backend	100.00% <ø> (ø)

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[read-the-docs-community]

Documentation build overview

📚 mdadash | 🛠️ Build #34042982 | 📁 Comparing a21b19b against latest (b885886)

  🔍 Preview build  

36 files changed · + 1 added · ± 35 modified

+ Added

• adding_custom_widgets.html

± Modified

• built_in_widgets.html
• getting_started.html
• index.html
• autosummary/mdadash.backend.analyses.acf.html
• autosummary/mdadash.backend.analyses.com_distance.html
• autosummary/mdadash.backend.analyses.contacts.html
• autosummary/mdadash.backend.analyses.custom_code.html
• autosummary/mdadash.backend.analyses.dssp.html
• autosummary/mdadash.backend.analyses.energies.html
• autosummary/mdadash.backend.analyses.helix_analysis.html
• and 25 more...