build man pages consistently across libs

[comick]

This wants to address #827 by adopting the exact same filtering found at https://github.com/podman-container-tools/podman/blob/63cf53d26a579ca8891d83f68fbaa1bf9c957a20/Makefile#L583.

Note that, as of PR creation, the only affected link is found at

container-libs/common/docs/containers.conf.5.md

Line 339 in 4e1a536

For example it can be set to `bridge`, `host`, `none`, `pasta` and more, see the [podman-create(1)](https://docs.podman.io/en/latest/markdown/podman-create.1.html#network-mode-net)

though it should then be safer to add more, if merged.

[packit-as-a-service]

Packit jobs failed. @containers/packit-build please check.

[mtrmac]

Thanks! The goal makes sense in general.

We have 2 other sets of man pages here. I suppose this should be centralized in a single shell script called from all those places.

---

I didn’t read the implementation any further than those 2 comments now.

[comick]

Thanks for the initial feedback.
Turning this into draft until agreed on the approach.

I'd like to do this in three steps:

1. (this PR) introduce a shared way to build man pages across common, image and storage libs
2. introduce the links cleaning preprocessing (like podman)
3. add links to man pages

As for this PR, /hack/man-docs.mk is introduced (podman seems to use this folder for similar helpers, but please advice) to provide reusable docs/install/clean targets, which are in turn used in common/image/storage own Makefiles. This is used in common and storage docs Makefiles. image had no docs Makefile (did man stuff in top-level Makefile), which is now introduced for consistency.

The man-docs.mk is where I'd later put the pre-processing/links-cleaning in step 2.

I tried to keep install target folders intact and tested by comparing the content of destination dir ($DEST_DIR/usr/share/man...) with and without my change and ensuring content matches