docs: add top-level intro and module-level intros (canonical#1320)

Docs: Ops: Add top-level intro, clean up ops.module intro in light of
that, flesh out ops.pebble and ops.testing intros a bit more

https://warthogs.atlassian.net/browse/CHARMTECH-183

- [x] "home page" library reference
- [x] ops module
- [x] ops.main entry point
- [x] ops.pebble module
- [x] ops.testing module

docs/index.rst

@@ -25,6 +25,24 @@ integrations with other services, and making the charm easier to test.
 
 .. _available on PyPI: https://pypi.org/project/ops/
 
+The `ops` library is a Python framework for writing and testing Juju charms.
+
+  See more: `Charm SDK documentation <https://juju.is/docs/sdk>`_
+
+The library provides:
+
+- :ref:`ops_main_entry_point`, used to initialise and run your charm;
+- :ref:`ops_module`, the API to respond to Juju events and manage the application;
+- :ref:`ops_pebble_module`, the Pebble client, a low-level API for Kubernetes containers;
+- :ref:`ops_testing_module`, the framework for unit testing charms in a simulated environment;
+
+You can structure your charm however you like, but with the `ops` library, you
+get a framework that promotes consistency and readability by following best
+practices. It also helps you organise your code better by separating different
+aspects of the charm, such as managing the application’s state, handling
+integrations with other services, and making the charm easier to test.
+
+
 .. toctree::
    :hidden:
    :maxdepth: 2
@@ -44,6 +62,8 @@ ops
 
 .. _ops_main_entry_point:
 
+.. _ops_main_entry_point:
+
 ops.main entry point
 --------------------