[PR] Migrate to v3

[LuchoTurtle]

closes #15

This PR migrates this package to v3. It:

• removes contentlayer2 and switches to remark. Links are now validated with remark-validate-links
• updates the theme
• updates the src folder according to the migration guide at https://the-guild.dev/blog/nextra-3.
• updates dependencies
• removes unnecessary/unused files.
• updated the README and tutorial.md to add instances of the difference between versions.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (322f93e) to head (4f05979).
Report is 35 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##              main       #16    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files            3         4     +1     
  Lines          211        69   -142     
  Branches        30         1    -29     
==========================================
- Hits           211        69   -142     

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[LuchoTurtle]

@nelsonic this should be mergeable. The bottleneck was contentlayer2, which was replaced with our linting strategy in the A docs repo. I've documented both strategies in the tutorial.md file.

[nelsonic]

@LuchoTurtle looks good. 👌

[nelsonic]

Especially like that this PR has a net removal of lines from the project. 👌