Update documentation to include venv support.

Signed-off-by: Leander Stephen D'Souza <leanderdsouza1234@gmail.com>

Author: leander-dsouza

README.md

@@ -4,13 +4,33 @@ https://docs.nav2.org/
 This folder holds the source and configuration files used to generate the
 [Navigation2 documentation](https://docs.nav2.org) web site.
 
+
+### Native Installation
+
 Dependencies for Build:
 
 ``` bash
 sudo apt install python3-pip
 pip3 install -r requirements.txt
 ```
 
+### Installation in a Virtual Environment
+
+Install `pip` and `venv` if not already installed:
+
+``` bash
+sudo apt install python3-pip python3-venv
+```
+
+Create a virtual environment and install the dependencies:
+
+``` bash
+python3 -m venv venv
+source venv/bin/activate
+pip3 install -r requirements.txt
+```
+
+### Build the Docs
 Build the docs locally with `make html` and you'll find the built docs entry point in `_build/html/index.html`.
 
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