ENH: Remove unnecessary docs/requirements.txt

Remove unnecessary `docs/requirements.txt`: documentation dependencies
are listed in the `[doc]` section of the `pyproject.toml` file.

Use the `pip install .[doc]` command to install the package
dependencies, including the documentation packages, in the GitHub
workflow files.

.github/workflows/docs-build-pr.yml

@@ -40,7 +40,7 @@ jobs:
     - name: Install dependencies
       run: |
         pip install -U build hatch pip
-        pip install -r docs/requirements.txt
+        pip install .[doc]
         python -m hatch version | tail -n1
 
     - name: Build docs

.github/workflows/docs-build-update.yml

@@ -43,7 +43,7 @@ jobs:
     - name: Install dependencies
       run: |
         pip install -U build hatch pip
-        pip install -r docs/requirements.txt
+        pip install .[doc]
         python -m hatch version | tail -n1
 
     - name: Build docs