Chore: Split BUILDIT.md into smaller more maintainable files

[nelsonic]

At present the BUILDIT.md is several thousand lines long:

While it's great that we've collectively made an effort to document the MVP in such great detail.
Having a single file this large makes it difficult to maintain and extend
especially with the last section numbering being a bit of a moving target ...

Each time a new section is added before the "??. Run the Finished MVP App!" section, the number has to be incremented manually.

Proposal: shift the docs of the MVP from this repo to the /book repo.

Todo

• Create a new directory in the book project: /src/mvp
• Split the mvp/BUILTIT.md file into separate files in the new directory
  • Name the new files according to the section heading and add a numeric index to the start of the file
    similar to: https://github.com/dwyl/book/tree/main/src/auth
• Link to the new mvp section (in "the book") from the mvp repo.

Note: this is a priority-3 task. Only look at it once the features you're working on are complete.
Docs are super important for capturing/sharing knowledge so they shouldn't be neglected.
We are proactively writing great docs to encourage learning and contribution.
Nobody reading the BUILDIT.md file has "complained" that it is "too long" ...
But if we can get into the habit of extending the book repo with our docs we can gradually collectively write a book that other people can read to learn what we are building. 💭

[nelsonic]

Ref: https://dwyl.github.io/book/mvp/index.html

[nelsonic]

e.g: https://dwyl.github.io/book/mvp/16-lists.html