docs: refactor docs/spec to use mdbook

[samlaf]

Closes EDGA-821
Still a wip so leaving as DRAFT, but curious for people's opinion here.

Using https://rust-lang.github.io/mdBook/index.html to publish the spec and further technical documentation in one place.
Mdbook has mathjax support, but they somehow don't support $...$ notation, so we have to replace all of them with \\( \\) (not sure whether this is mathjax-specific syntax or if latex supports this syntax as well). I changed all the $s in overview.md but haven't touched the other pages yet.

To view the book, need to

cargo install mdbook
# checkout this branch
cd docs/spec
mdbook serve --open

Looks like

for now but we'll prob want to reorganize the overview into a more book-like format.

Why are these changes needed?

Checks

• I've made sure the tests are passing. Note that there might be a few flaky tests, in that case, please comment that they are not relevant.
• I've checked the new test coverage and the coverage percentage didn't drop.
• Testing Strategy
  • Unit tests
  • Integration tests
  • This PR is not tested :(