New TARDIS flowchart and cleaning up physics walkthrough

[isaacgsmith]

📝 Description

Type: 📝 documentation

Three main changes:

• Adding a new TARDIS flowchart to the physics walkthrough introduction.
• Moving the gamma ray deposition pages to their appropriate spot (and adding a bibtex entry to a missing citation in one of those pages).
• Fixing some issues with the light and matter page (specifically renaming kappa to alpha for opacity, and referencing the page where necessary).

📌 Resources

Main edited page:
https://tardis-sn.github.io/tardis/pull/2111/physics/intro/index.html

Other edited sections:
https://tardis-sn.github.io/tardis/pull/2111/contributing/in_progress/index.html#simulate-supernovae-in-the-nebular-phase
https://tardis-sn.github.io/tardis/pull/2111/physics/intro/light_and_matter.html#opacity-and-optical-depth
https://tardis-sn.github.io/tardis/pull/2111/physics/montecarlo/propagation.html#physical-interactions

🚦 Testing

How did you test these changes?

• Testing pipeline
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• My changes can't be tested (explain why)
  Docs built on GitHub.

☑️ Checklist

• I requested two reviewers for this pull request
• I updated the documentation according to my changes
• I built the documentation by applying the build_docs label

Note: If you are not allowed to perform any of these actions, ping (@) a contributor.

[codecov]

Codecov Report

Merging #2111 (ee49ddb) into master (973f9a5) will not change coverage.
The diff coverage is n/a.

❗ Current head ee49ddb differs from pull request most recent head 29aa332. Consider uploading reports for the commit 29aa332 to get more accurate results

@@           Coverage Diff           @@
##           master    #2111   +/-   ##
=======================================
  Coverage   61.45%   61.45%           
=======================================
  Files          75       75           
  Lines        8617     8617           
=======================================
  Hits         5296     5296           
  Misses       3321     3321           

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