Docs: macOS installation instructions fail on modern macOS (PEP 668 / externally-managed-environment)

**Issue details**
I was following the macOS installation guide today on my MacBook Air (Sonoma), and the `pip install` command failed with the `externally-managed-environment` error.

**Screenshot 1: The current documentation**

**Version**
Latest macOS (Sonoma/Sequoia)

**Platform**
[X] Mac

**Description**
The current macOS documentation recommends using `python3 -m pip install ... --user` or `brew install python`. This fails on modern macOS versions because of **PEP 668** system protection, which prevents pip from modifying the system-managed Python environment.

**Steps to Reproduce**
1. On a fresh macOS installation (Sonoma 14.x or newer).
2. Run the command from the docs: `python3 -m pip install mavproxy pymavlink --user --upgrade`.

**Actual Behavior**
The installation is blocked by the OS with the following error:

**Screenshot 2: The terminal error**

**Proposed Solution**
The documentation should be updated to explicitly recommend installing within a virtual environment (`venv`) or using `pipx`.

**Recommended update for the Mac section:**
```bash
# Create a virtual environment to avoid PEP 668 errors
python3 -m venv venv
source venv/bin/activate

# Install inside the virtual environment
pip install MAVProxy pymavlink

Start with the macOS installation guide's Mac section and find the documented `pip install` command using `--user`. Update the instructions to recommend a virtual environment or pipx, using the proposed venv commands as a guide. Done means a fresh modern macOS installation has a documented path that avoids the PEP 668 externally-managed-environment error.

Written by the indexing model from the issue text.