Remove properly tracked config file from .gitignore & add support for skipping of link-checking

[marxin]

The file is part of the git history and is a configuration file.

Fixes: #2018

[camelid]

I think the reason this is ignored is that we often want to change book.toml locally to turn off link-checking. I'm not sure it makes sense to remove it from the gitignore unless there's another way to disable link-checking locally.

[marxin]

Sure, that makes sense and I've just added the support. What do you think about it now?

[marxin]

@camelid May I please ping this?

[marxin]

@camelid: A gentle ping.