Improvements for “Building the manual with Sphinx” (#6171)

* Streamline “Building the manual with Sphinx”

The goal of this commit is to make the instructions in “Building the
manual with Sphinx” easier to follow. It does so by

1. Removing items from the prerequisites list that would get installed by
   requirements.txt anyway.
2. Adding items to the prerequisites list that were missing.
3. Making it possible to follow the instructions by reading them from
   top to bottom without jumping around.
4. Putting the meat of the instructions into an ordered list (this will
   make it easier for readers to not lose their place).

* Minor English fix for “Building the manual with Sphinx”

community/contributing/building_the_manual.rst

@@ -7,39 +7,62 @@ This page explains how to build a local copy of the Godot manual using the
 Sphinx docs engine. This allows you to have local HTML files and build the
 documentation as a PDF, EPUB, or LaTeX file, for example.
 
-To get started, you need to:
+Before you get started, make sure that you have:
 
-1. Clone the `godot-docs repository <https://github.com/godotengine/godot-docs/>`__.
-2. Install `Sphinx <https://www.sphinx-doc.org/>`__
-3. To build the docs as HTML files, install the `readthedocs.org theme
-   <https://github.com/snide/sphinx_rtd_theme>`__.
-4. Install the Sphinx extensions defined in the `godot-docs repository
-   <https://github.com/godotengine/godot-docs/>`__ ``requirements.txt`` file.
+- `Git <https://git-scm.com/>`_
+- `make <https://www.gnu.org/software/make/>`_ (unless you’re using Windows)
+- `Python <https://www.python.org/>`_ 3
 
-We recommend using `pip <https://pip.pypa.io>`__, Python’s package manager to
-install all these tools. It comes pre-installed with `Python
-<https://www.python.org/>`__. Ensure that you install and use Python 3. Here are
-the commands to clone the repository and then install all requirements.
+.. note:: Python 3 should come with the ``pip3`` command. You may need to write
+    ``python3 -m pip`` (Unix) or  ``py -m pip`` (Windows) instead of ``pip3``.
+    If both approaches fail, `make sure that you have pip3 installed
+    <https://pip.pypa.io/en/stable/installation/>`__.
 
-.. note:: You may need to write ``python3 -m pip`` (Unix) or  ``py -m pip`` (Windows) instead of ``pip3``.
-          If both approaches fail, `check that you have pip3 installed <https://pip.pypa.io/en/stable/installation/>`__.
+1.  *(Optional)* Set up a virtual environment. Virtual environments prevent
+    potential conflicts between the Python packages in ``requirements.txt`` and
+    other Python packages that are installed on your system.
 
-.. code:: sh
+    a.  Create the virtual environment:
 
-    git clone https://github.com/godotengine/godot-docs.git
-    pip3 install -r requirements.txt
+        - On Windows, run ``py -m venv godot-docs-venv``
+        - On other platforms, run ``python3 -m venv godot-docs-venv``
 
+    b.  Activate the virtual environment:
 
-With the programs installed, you can build the HTML documentation from the root
-folder of this repository with the following command:
+        - On Windows, run ``godot-docs-venv\Scripts\activate.bat``
+        - On other platforms, run ``source godot-docs-venv/bin/activate``
 
-.. code:: sh
+    c.  *(Optional)* Update pre-installed packages:
+
+        - On Windows, run ``py -m pip install --upgrade pip setuptools``
+        - On other platforms, run ``pip3 install --upgrade pip setuptools``
+
+2.  Clone the docs repo:
+
+    .. code:: sh
+
+        git clone https://github.com/godotengine/godot-docs.git
+
+3.  Change directory into the docs repo:
+
+    .. code:: sh
+
+        cd godot-docs
+
+4.  Install the required packages:
 
-    # On Linux and macOS
-    make html
+    .. code:: sh
 
-    # On Windows, you need to execute the ``make.bat`` file instead.
-    make.bat html
+        pip3 install -r requirements.txt
+
+5.  Build the docs:
+
+    .. code:: sh
+
+        make html
+
+.. note:: On Windows, that command will run ``make.bat`` instead of GNU Make (or
+          an alternative).
 
 If you run into errors, you may try the following command:
 
@@ -65,7 +88,7 @@ of files.
 You can then browse the documentation by opening ``_build/html/index.html`` in
 your web browser.
 
-In case you of a ``MemoryError`` or ``EOFError``, you can remove the
+If you get a ``MemoryError`` or ``EOFError``, you can remove the
 ``classes/`` folder and run ``make`` again. This will drop the class references
 from the final HTML documentation but will keep the rest intact.
 
@@ -87,18 +110,3 @@ You can also specify a list of files to build, which can greatly speed up compil
 .. code:: sh
 
   sphinx-build -b html ./ _build classes/class_node.rst classes/class_resource.rst
-
-Building with Sphinx and virtualenv
------------------------------------
-
-If you want your Sphinx installation scoped to the project, you can install
-sphinx-build using virtualenv. To do so, run this command from this repository's
-root folder:
-
-.. code:: sh
-
-   virtualenv --system-site-packages env/
-   . env/bin/activate
-   pip3 install -r requirements.txt
-
-Then, run ``make html`` as shown above.