docs: Add CUE walkthrough and module dev guides

[stefanprodan]

This PR adds a docs section dedicated to learning CUE and writing Timoni modules from scratch.

Preview:

• https://timoni.sh/cue/walkthrough/
• https://timoni.sh/cue/module/initialization/
• https://timoni.sh/cue/module/custom-resources/
• https://timoni.sh/cue/module/test-jobs/ and more

closes: #251 fixes: #313