fix(systemd): respect XDG_CONFIG_HOME

Use `XDG_CONFIG_HOME` instead of `~/.config` if set.

Fixes #79.

Remove reference to pkg_resources

Drop 3.8 in CI

Add Documentation= to systemd unit file

Reference: https://www.freedesktop.org/software/systemd/man/systemd.unit.html#%5BUnit%5D%20Section%20Options

Fix licensing

Force poetry-core 1.0.0

Avoiding python-poetry/poetry#3650

Only on push on main

Add publish workflow, refine main trigger

Merge branch 'issue-15'

Closes #15

Release 0.2.1

Bugfix release merging 872075a