docs: Add instructions for building documentation

Since the instructions suggest to create a local environment called `.venv`,
it is  added to `.gitignore` and excluded from the documentation
paths in `conf.py`.

Documentation/dev/build_documentation.md

 # Building documentation
 
-```{tip}
-For complete build instructions see [here](build.md).
+This section outlines how to locally build both the user and developer guides and the C++ API
+documentation for VTK.
+
+## User and developer guides
+
+VTK's user and developer guides are automatically built and deployed to https://docs.vtk.org
+every time the `master` branch is updated by leveraging the integration with the
+_Read the Docs_ service.
+
+To locally build the documentation:
+
+::::::{tab-set}
+
+:::::{tab-item} Without VTK build tree
+
+1. [Download](/build_instructions/build.md#obtaining-the-sources) the VTK sources.
+
+2. Create and activate a virtual environment.
+
+::::{tab-set}
+
+:::{tab-item} Linux/macOS
+:sync: linux-or-macos
+
+```shell
+cd Documentation/docs
+
+python3 -m venv .venv
+source .venv/bin/activate
+```
+
+:::
+
+:::{tab-item} Windows
+:sync: windows
+
+```bat
+cd Documentation\docs
+
+py -m venv .venv
+.\.venv\Scripts\activate
+```
+
+`py -m venv` executes venv using the latest Python interpreter you have installed.
+For more details, read the [Python Windows launcher](https://docs.python.org/3/using/windows.html#launcher) docs.
+
+:::
+
+::::
+
+3. Install dependencies using pip.
+
+::::{tab-set}
+
+:::{tab-item} Linux/macOS
+:sync: linux-or-macos
+
+```shell
+python3 -m pip install -r requirements.txt
+```
+
+:::
+
+:::{tab-item} Windows
+:sync: windows
+
+```bat
+py -m pip install -r requirements.txt
+```
+
+:::
+
+::::
+
+4. Build the documentation as web pages.
+
+```shell
+make html
+```
+
+5. Open `_build/html/index.html` in a web browser.
+
+::::{tab-set}
+
+:::{tab-item} Linux
+```shell
+xdg-open _build/html/index.html
+```
+
+:::
+
+:::{tab-item} macOS
+
+```shell
+open _build/html/index.html
+```
+
+:::
+
+:::{tab-item} Windows
+:sync: windows
+
+```bat
+start _build\html\index.html
 ```
 
-The following targets are used to build documentation for VTK:
+:::
+
+::::
+
+:::::
+
+:::::{tab-item} With VTK build tree
+
+1. [Download](/build_instructions/build.md#obtaining-the-sources) VTK sources.
+
+2. [Configure](/build_instructions/build.md#configure) VTK by setting the `VTK_BUILD_SPHINX_DOCUMENTATION`
+   option to `ON`.
+
+2. Build the `SphinxDoc` target.
+
+:::::
+
+::::::
+
+:::{hint}
+Automatic build of preview documentation each time a merge request is submitted is not yet
+supported due to [limitation](https://docs.readthedocs.io/en/stable/guides/pull-requests.html#limitations)
+of the _Read The Docs_ service that does not yet support self-hosted GitLab deployment.
+
+Solutions to address this are being discussed in https://github.com/readthedocs/readthedocs.org/issues/9464.
+:::
+
+## C++ API documentation
+
+The C++ API documentation is built and uploaded to https://vtk.org/doc/nightly/html/index.html
+when the `master` branch is updated.
+
+To locally build the documentation:
+
+1. Install Doxygen
+
+2. [Download](/build_instructions/build.md#obtaining-the-sources) the VTK sources.
+
+3. [Configure](/build_instructions/build.md#configure) VTK by setting the `VTK_BUILD_DOCUMENTATION` option to `ON`.
+
+3. Build the `DoxygenDoc` target.
+
+## Targets
+
+After configuring the VTK using CMake, the following targets may be used to
+build documentation for VTK:
 
   * `DoxygenDoc` - build the doxygen documentation from VTK's C++ source files
   (`VTK_BUILD_DOCUMENTATION` needs to be `ON` for the target to exist).

Documentation/docs/.gitignore

+.venv
 _build

Documentation/docs/conf.py

@@ -60,7 +60,7 @@ templates_path = ["_templates"]
 # List of patterns, relative to source directory, that match files and
 # directories to ignore when looking for source files.
 # This pattern also affects html_static_path and html_extra_path.
-exclude_patterns = ["_build", "Thumbs.db", ".DS_Store", "README.md"]
+exclude_patterns = ["_build", ".venv", "Thumbs.db", ".DS_Store", "README.md"]
 
 todo_include_todos = True