flake: add templates

[MaxCan-Code]

Description

Add example flakes from docs/nix-flakes.adoc as templates. Format standalone flake in docs/nix-flakes.adoc

Checklist

• [x] Change is backwards compatible.

• [x] Code formatted with ./format.

• [x] Code tested through nix-shell --pure tests -A run.all.

• [ ] Test cases updated/added. See example.

• [x] Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  See CONTRIBUTING for more information and recent commit messages for examples.

• If this PR adds a new module

  • [ ] Added myself as module maintainer. See example.

  • [ ] Added myself and the module files to .github/CODEOWNERS.

[stale[bot]]

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[ShamrockLee]

It would be more maintainable to rebase onto the master branch instead of making an extra commit to replay the changes on the master branch.

[teto]

This Pr is a good idea

[MaxCan-Code]

The formatting check should pass now