README: Expand Local build instructions

This better covers how to build on Debian and Ubuntu.

Signed-off-by: Richard Laager <rlaager@wiktel.com>

README.rst

@@ -1,19 +1,39 @@
 .. image:: docs/_static/img/logo/480px-Open-ZFS-Secondary-Logo-Colour-halfsize.png
+.. highlight:: sh
 
 OpenZFS Documentation
 =====================
 
 Public link: https://openzfs.github.io/openzfs-docs/
 
-Local build
+Building Locally
------------
+----------------
+
+Install Prerequisites
+~~~~~~~~~~~~~~~~~~~~~
+
+Debian 11 (“testing”) / Ubuntu 20.04 or later::
+
+   sudo apt install python3-sphinx python3-sphinx-issues python3-sphinx-rtd-theme
+
+Debian 10 (”Buster”) and earlier::
+
+   sudo apt install pip3
+   pip3 install -r requirements.txt
+   # Add ~/.local/bin to your $PATH, e.g. by adding this to ~/.bashrc:
+   PATH=$HOME/.local/bin:$PATH
+
+Other distros::
+
+   pip install -r requirements.txt
+   # Add ~/.local/bin to your $PATH, e.g. by adding this to ~/.bashrc:
+   PATH=$HOME/.local/bin:$PATH
+
+Build
+~~~~~
 
 ::
 
-  cd ./docs/
+   cd docs
-  # install dependencies:
+   make html
-  pip install -r requirements.txt
+   # HTML files will be generated in: _build/html
-  # run `sphinx` to build pages
-  make html
-  # html files will be generated in:
-  cd ./docs/_build/html/