docs: Missing requirement "python3-devel"

Following the installation instructions at https://www.gdbgui.com/installation/ for "pipx installation", the installer will apparently attempt to compile something.

Alas, on (probably) many systems Python.h will not be found, resulting in the error message `Python.h: No such file or directory` at installation time.

This can be fixed by providing the Python header files to the build environment.

**To Reproduce**
Steps to reproduce the behavior:
* simple(!) Fedora 34 installation (which has Python 3.9.x)
* `python3 -m pip install --user pipx`
* `pipx install gdbgui`

Resolve by adding text to the documentation mentioning that Python development tooling is required (will be required before attempting to install).

For Fedora 34, `sudo dnf install python3-devel` will suffice; re-running `pipx install gdbgui` will then complete the installation process successfully.