Support {bslib} in Rhino

[kamilzyla]

Changes

Closes #595. Implements the changes proposed in Design Doc: {bslib} in Rhino:

1. Add sass: custom configuration option and handle it in build_sass().
2. Attach favicon and minified files only if they exist (check each time the app is reloaded).
3. Update documentation (how-to, rhino.yml, changelog).

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 44.44444% with 10 lines in your changes missing coverage. Please review.

Project coverage is 20.29%. Comparing base (07f2f56) to head (1f061c3).

❗ Current head 1f061c3 differs from pull request most recent head 565ef98

Please upload reports for the commit 565ef98 to get more accurate results.

Files	Patch %	Lines
R/app.R	57.14%	6 Missing ⚠️
R/tools.R	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #596      +/-   ##
==========================================
- Coverage   20.65%   20.29%   -0.36%     
==========================================
  Files           9        9              
  Lines         460      468       +8     
==========================================
  Hits           95       95              
- Misses        365      373       +8     

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[TymekDev]

Code looks good! I haven't read the vignette, though (sick leave got me 😖) - I'll leave that to @jakubnowicki

[jakubnowicki]

Nicely done. Please resolve my comments regarding the guide. Also, please check the discussion in the design doc. I'm ok with merging the PR as is, but I would love to hear your thoughts.