Document all environment variables Pwndbg depends on

[disconnect3d]

We should document all envvars Pwndbg depends on.

Those are:

PATH
EDITOR, VISUAL -- used by cymbol command
HOME, XDG_CACHE_HOME -- used by lib.tempfile

PWNDBG_VENV_PATH
PWNDBG_DISABLE_COLORS

OPENAI_API_KEY, ANTHROPIC_API_KEY -- for ai command

GITHUB_ACTIONS, RUN_FLAKY -- used by tests_commands.py

PWNDBG_PROFILE -- for benchmarking
USE_PDB -- used in tests
PWNDBG_LAUNCH_TEST -- used by tests

PWNDBG_ARCH, PWNDBG_KERNEL_TYPE, PWNDBG_KERNEL_VERSION -- used by gdblib kernel tests

SPHINX -- used by docs/source/conf.py -- probably to be removed?

Additionally, Pwndbg sets PWNLIB_NOTERM=1 to work around some Pwntools stuff