docs: tox -e docs-deps to compile requirements.txt (#1172)

Separate pip-compile into a separate env `tox -e docs-deps`.

HACKING.md

@@ -26,6 +26,9 @@ tox -e unit test/test_charm.py
 # Format the code using isort and autopep8
 tox -e fmt
 
+# Compile the requirements.txt file for docs
+tox -e docs-deps
+
 # Generate a local copy of the Sphinx docs in docs/_build
 tox -e docs
 
@@ -198,6 +201,13 @@ tox -e docs
 open docs/_build/html/index.html
 ```
 
+If dependencies are updated in `pyproject.toml`, you can run the following command
+before generating docs to recompile the `requirements.txt` file used for docs:
+
+```sh
+tox -e docs-deps
+```
+
 ## How to Pull in Style Changes
 
 The documentation uses Canonical styling which is customised on top of the [Furo Sphinx theme](https://github.com/pradyunsg/furo). The easiest way to pull in Canonical style changes is by using the Canonical documentation starter pack, see [docs](https://canonical-starter-pack.readthedocs-hosted.com/) and [repository](https://github.com/canonical/sphinx-docs-starter-pack).

tox.ini

@@ -23,11 +23,16 @@ passenv =
   PATH
   MODEL_SETTINGS
 
+[testenv:docs-deps]
+description = Compile the requirements.txt file for docs
+deps = pip-tools
+commands =
+    pip-compile --extra=docs -o docs/requirements.txt pyproject.toml
+
 [testenv:docs]
 description = Build the Sphinx docs
 deps = pip-tools
 commands_pre =
-    pip-compile --extra=docs -o docs/requirements.txt pyproject.toml
     pip-sync {toxinidir}/docs/requirements.txt
 commands =
     sphinx-build -W --keep-going docs/ docs/_build/html