docs: add READMEs to packages

[georgexu99]

Co-authored-by: Keeley Hammond vertedinde@electronjs.org

This PR adds more detailed READMEs to each Forge module.

Deployed live here at https://add-readmes--marvelous-choux-77d0b7.netlify.app/

• I have read the contribution documentation for this project.
• I agree to follow the code of conduct that this project follows, as appropriate.
• The changes are appropriately documented (if applicable).
• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:

[codecov]

Codecov Report

Merging #2986 (f6b2d36) into main (8fa40d4) will decrease coverage by 0.47%.
The diff coverage is n/a.

❗ Current head f6b2d36 differs from pull request most recent head 622649b. Consider uploading reports for the commit 622649b to get more accurate results

@@            Coverage Diff             @@
##             main    #2986      +/-   ##
==========================================
- Coverage   74.27%   73.80%   -0.48%     
==========================================
  Files          67       67              
  Lines        2185     2168      -17     
  Branches      438      436       -2     
==========================================
- Hits         1623     1600      -23     
- Misses        354      361       +7     
+ Partials      208      207       -1     

Impacted Files	Coverage Δ
packages/maker/snap/src/MakerSnap.ts	93.33% <0.00%> (-6.67%)	⬇️
packages/plugin/base/src/Plugin.ts	57.14% <0.00%> (-5.36%)	⬇️
packages/maker/dmg/src/MakerDMG.ts	95.00% <0.00%> (-5.00%)	⬇️
packages/maker/wix/src/MakerWix.ts	81.81% <0.00%> (-4.55%)	⬇️
packages/maker/deb/src/MakerDeb.ts	90.90% <0.00%> (-4.55%)	⬇️
packages/maker/rpm/src/MakerRpm.ts	90.90% <0.00%> (-4.55%)	⬇️
packages/publisher/base/src/Publisher.ts	62.50% <0.00%> (-4.17%)	⬇️
packages/maker/base/src/Maker.ts	69.69% <0.00%> (-3.84%)	⬇️
packages/maker/appx/src/MakerAppX.ts	60.34% <0.00%> (-1.73%)	⬇️
packages/maker/squirrel/src/MakerSquirrel.ts	76.19% <0.00%> (-1.09%)	⬇️
... and 42 more

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