Add beginner session and scala3 codec doc

[Quafadas]

I wrote out the documentation, which I believe may have helped me to get Skunk in the right place in my own programs.

This may be undesirable for several reasons.

• It may not be best practice - I'm not a confident user of skunk,
• It may be both too specific (an http4s app)
• As well as too broad - should skunk docs concern themselves with helping with app design?

What I'm saying is that I would not be offended, if this documentation PR was not merged - but at least I tried to contribute my learning back.

[GitsMcGee]

This documentation would have helped me when I first got started, so thank you!

[codecov]

Codecov Report

Merging #791 (21e9c27) into main (f826178) will not change coverage.
The diff coverage is n/a.

❗ Current head 21e9c27 differs from pull request most recent head d59c957. Consider uploading reports for the commit d59c957 to get more accurate results

@@           Coverage Diff           @@
##             main     #791   +/-   ##
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  Coverage   84.93%   84.93%           
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  Files         126      126           
  Lines        1719     1719           
  Branches      117      117           
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  Hits         1460     1460           
  Misses        259      259           

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