docs: clarify documentation about local environment setup (#13763)

This change makes the local environment setup documentation clearer and
easier to understand

## Checklist
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- The PR description articulates the motivation for the change
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- The PR description notes risks associated with the change, if any
- Newly-added code is easy to change
- The change follows the [library release note
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- The change includes or references documentation updates if necessary
- Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist
- [ ] Reviewer has checked that all the criteria below are met 
- Title is accurate
- All changes are related to the pull request's stated goal
- Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- Testing strategy adequately addresses listed risks
- Newly-added code is easy to change
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implications of this PR as reported in the benchmarks PR comment
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[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)

Author: emmettbutler

docs/contributing-testing.rst

@@ -29,37 +29,23 @@ of code organization.
 How do I run the test suite?
 ----------------------------
 
-We assume you have `docker <https://www.docker.com/products/docker>`_ installed.
+Install and run `docker <https://www.docker.com/products/docker>`_ .
 
-In addition, you will need `riot <https://ddriot.readthedocs.io/en/latest/>`_ and `hatch <https://hatch.pypa.io/latest/>`_.
+This repo includes a Docker container definition that provides a prebuilt test environment.
+You can access it by running
 
 .. code-block:: bash
 
-    $ pip install riot==0.20.1
+    $ scripts/ddtest
 
-Refer `hatch install <https://hatch.pypa.io/latest/install/>`_ for installation instructions
+Some of our test suites are managed with Riot, others with Hatch.
 
-Some of our test environments are managed with Riot, others with Hatch.
-
-For riot environments, you can run:
+You can run riot and hatch commands in the test runner container with commands like these:
 
 .. code-block:: bash
 
     $ scripts/ddtest riot run -p 3.10
-
-This command runs the entire test suite, which is probably not what you want to do.
-
-For hatch environments, you can run:
-
-.. code-block:: bash
-
-    $ hatch run lint:style
-
-If you make a change to the `hatch.toml` or library dependencies, be sure to remove environments before re-running:
-
-.. code-block:: bash
-
-    $ hatch env remove <ENV> # or hatch env prune
+    $ scripts/ddtest hatch run lint:style
 
 
 How do I run only the tests I care about?
@@ -188,6 +174,16 @@ environment object.
    ``for h in `riot list --hash-only "^${VENV_NAME}$"`; do rm .riot/requirements/${h}.txt; done; scripts/compile-and-prune-test-requirements``
 4. Commit the resulting changes to the ``.riot`` directory, and open a pull request against the trunk branch.
 
+Why isn't my hatch config change taking effect?
+-----------------------------------------------
+
+If you make a change to the `hatch.toml` or library dependencies, be sure to remove environments before re-running:
+
+.. code-block:: bash
+
+    $ scripts/ddtest hatch env remove <ENV> # or hatch env prune
+
+
 What do I do when my pull request has failing tests unrelated to my changes?
 ----------------------------------------------------------------------------