[ENH] - Add architecture.md file

[trallard]

As this repo is due to evolve and might get a tad complex to see what is in it from a top level we need an architecture.md file detailing the contents.

Some examples and resources:

• https://github.com/rust-lang/rust-analyzer/blob/d7c99931d05e3723d878bea5dc26766791fa4e69/docs/dev/architecture.md
• https://matklad.github.io/2021/02/06/ARCHITECTURE.md.html
• https://github.com/executablebooks/sphinx-book-theme/blob/master/ARCHITECTURE.md

Should follow #3