feat: add `raw_sql` API

[abonander]

This is meant to be much easier to discover than the current approach of directly invoking Executor methods.

In addition, I'm improving documentation for the query*() functions across the board.

This also deprecates .execute_many() and .fetch_many() on these types because only the SQLite driver actually ever supported multiple statements in a single query string with the prepared statement interface, and that's just a consequence of the fact that it handles all statements as prepared statements.

[jplatte]

Not sure how useful this piece of feedback is, but from what I could gather when skimming the diff, this (mainly the updated documentation) is a massive QoL improvement and while I didn't personally need it, I can see how it will make SQLx a lot more approachable for some people!

[abonander]

Changed some tests to use raw_sql to make sure it works.