Add basic docs for the parser crate (#11199)

## Summary

This PR adds a basic README for the `ruff_python_parser` crate and
updates the CONTRIBUTING docs with the fuzzer and benchmark section.

Additionally, it also updates some inline documentation within the
parser crate and splits the `parse_program` function into
`parse_single_expression` and `parse_module` which will be called by
matching against the `Mode`.

This PR doesn't go into too much internal detail around the parser logic
due to the following reasons:
1. Where should the docs go? Should it be as a module docs in `lib.rs`
or in README?
2. The parser is still evolving and could include a lot of refactors
with the future work (feedback loop and improved error recovery and
resilience)

---------

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

crates/ruff_python_ast/src/nodes.rs

@@ -3615,6 +3615,9 @@ impl Deref for TypeParams {
     }
 }
 
+/// A suite represents a [Vec] of [Stmt].
+///
+/// See: <https://docs.python.org/3/reference/compound_stmts.html#grammar-token-python-grammar-suite>
 pub type Suite = Vec<Stmt>;
 
 /// The kind of escape command as defined in [IPython Syntax] in the IPython codebase.