build documentation with correct path for ssh_filter_btrbk

Hey.

I think it would be nice if at least here:
https://github.com/digint/btrbk/blob/af2d7b2c99bac7c5af2f9b2c5f0b1947be525597/doc/ssh_filter_btrbk.1.asciidoc?plain=1#L48
the pathname would match that where it's actually installed.

Does `asciidoc` provide any replacement mechanism or should I make something with sed and rename the file to `.in`?

Possibly also here:
https://github.com/digint/btrbk/blob/af2d7b2c99bac7c5af2f9b2c5f0b1947be525597/README.md?plain=1#L554-L561

OTOH, that should be directly readable, so maybe just hardcode `/usr/share/btrbk/scripts/ssh_filter_btrbk.sh`.

Cheers,
Chris.

Inspect doc/ssh_filter_btrbk.1.asciidoc and the referenced README.md section first, then trace how the documentation is built and where ssh_filter_btrbk.sh is installed. Done means the generated documentation and README both show the correct installed pathname without making the source text misleading.

Written by the indexing model from the issue text.