docs: Add FreeBSD to installation.md

[spmzt]

Summary of changes

Added the pipx installation under FreeBSD using pkg(8) and pip.

Test plan

Tested by running

[spmzt@cornelia] [~] % doas pkg install -y py311-pipx
Updating FreeBSD repository catalogue...
FreeBSD repository is up to date.
All repositories are up to date.
The following 3 package(s) will be affected (of 0 checked):

New packages to be INSTALLED:
	py311-pipx: 1.7.1
	py311-platformdirs: 4.3.6
	py311-userpath: 1.9.2

Number of packages to be installed: 3

The process will require 1 MiB more space.
212 KiB to be downloaded.
[1/3] Fetching py311-userpath-1.9.2.pkg: 100%   20 KiB  20.6kB/s    00:01    
[2/3] Fetching py311-platformdirs-4.3.6.pkg: 100%   32 KiB  32.3kB/s    00:01    
[3/3] Fetching py311-pipx-1.7.1.pkg: 100%  161 KiB 164.6kB/s    00:01    
Checking integrity... done (0 conflicting)
[1/3] Installing py311-platformdirs-4.3.6...
[1/3] Extracting py311-platformdirs-4.3.6: 100%
[2/3] Installing py311-userpath-1.9.2...
[2/3] Extracting py311-userpath-1.9.2: 100%
[3/3] Installing py311-pipx-1.7.1...
[3/3] Extracting py311-pipx-1.7.1: 100%
[spmzt@cornelia] [~] % pip install --user pipx
Requirement already satisfied: pipx in /usr/local/lib/python3.11/site-packages (1.7.1)
Requirement already satisfied: argcomplete>=1.9.4 in /usr/local/lib/python3.11/site-packages (from pipx) (3.5.3)
Requirement already satisfied: packaging>=20 in /usr/local/lib/python3.11/site-packages (from pipx) (24.2)
Requirement already satisfied: platformdirs>=2.1 in /usr/local/lib/python3.11/site-packages (from pipx) (4.3.6)
Requirement already satisfied: userpath!=1.9,>=1.6 in /usr/local/lib/python3.11/site-packages (from pipx) (1.9.2)
Requirement already satisfied: click in /usr/local/lib/python3.11/site-packages (from userpath!=1.9,>=1.6->pipx) (8.1.7)
[spmzt@cornelia] [~] % pipx ensurepath
Success! Added /home/spmzt/.local/bin to the PATH environment variable.

Consider adding shell completions for pipx. Run 'pipx completions' for instructions.

You will need to open a new terminal or re-login for the PATH changes to take effect. Alternatively, you can source your shell's config file with e.g. 'source ~/.bashrc'.

Otherwise pipx is ready to go! ✨ 🌟 ✨