Revamp docs and add page on IDL-fiasco comparisons

[wtbarnes]

Fixes #255
Fixes #208
Fixes #215

Remaining TODOs

• Flesh out the atomic database topic guide more
• Add the fiasco-IDL comparisons as a topic guide page
• Add the topic guide page to the landing page as a new panel
• Figure out organization of the panels on the landing page
• Fix the doctest
• Add more test files to the database used on RTD

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (9d531da) 92.63% compared to head (10542c5) 92.30%.

Files	Patch %	Lines
fiasco/fiasco.py	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #259      +/-   ##
==========================================
- Coverage   92.63%   92.30%   -0.34%     
==========================================
  Files          38       38              
  Lines        2770     2780      +10     
==========================================
  Hits         2566     2566              
- Misses        204      214      +10     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[wtbarnes]

The last thing to do is make sure we have all the needed files for computing the rates (ionization and recombination) in the test file list. Currently, the compute rates are not showing up on the rendered docs because those files are missing from the test database.