docs: Add tket2-py module docstring (#539)

Just copied the info from the README.

This is mainly intended to trigger a release-please PR for the
component.

tket2-py/tket2/__init__.py

@@ -1,3 +1,14 @@
+"""TKET2 is an open source quantum compiler developed by Quantinuum.
+
+Central to TKET2's design is its hardware agnosticism which allows researchers
+and quantum software developers to take advantage of its powerful compilation
+for many different quantum architectures.
+
+This is the Python front-end for TKET2, providing a high-level interface for
+working with quantum circuits. See also the Rust library with the same name on
+[crates.io](https://crates.io/crates/tket2).
+"""
+
 from . import circuit, ops, optimiser, passes, pattern, rewrite
 
 __all__ = ["circuit", "ops", "optimiser", "passes", "pattern", "rewrite"]