Fixes waycrate#101

Why?

- no build.rs that is invoked on _any_ `cargo build/run`
- no extra dev dependency
- no intermediate `docs/*.out` files
- 3 lines of bash instead
- maximum gzip compression (-9/--best)
- docs will be built only when needed,
    i.e. on `make docs` or `make install`

To quickly check:

```sh
make TARGET_DIR=/tmp/wayshot MAN{1,7}_DIR=/tmp/wayshot install
```

Shows exactly what is going on.

Also fixes a minor issue of not cleaning `./docs/*.7.gz`

For `DEBIAN_FRONTEND` environment variable see `man 7 debconf`:

- https://manpages.debian.org/bookworm/debconf-doc/debconf.7.en.html#noninteractive