Add admonitions where appropriate

Some examples of where we can use it:

- [ ] [Profile section](https://docs.ankiweb.net/profiles.html#profiles) warns user they cannot sync multiple profiles with one account.

- [ ] The [security warning](https://docs.ankiweb.net/math.html#security-warning) for latex.

- [ ] Look for other things where we can use it.

~~Refer to mdBook docs for how you do it: https://rust-lang.github.io/mdBook/format/mdbook.html#classwarning~~

Actually, we can just use this third-party preprocessor: https://github.com/tommilligan/mdbook-admonish

It gives us more features/flexibility like we can use it to insert FAQs into normal pages (which looks dope if you have seen other sites like that before).