ci/gha: add space-at-eol check, fix existing issues

[kolyshkin]

This adds a new CI check that there's no extra whitespace at EOL, and fixes all existing cases (not much).

Mostly doing it because my $EDITOR is configured to remove extra whitespace at EOL for most file types, and I don't want to introduce extra hunks when working on the docs.

The failed run looks like this:

MAINTAINERS_GUIDE.md:53:* Step 3: Accept (`LGTM`) or refuse a pull request. The relevant maintainers do 
NOTICE:11:United States and other governments.  
....
libcontainer/nsenter/README.md:12:So every time we  import package `nsenter`, the C code function `nsexec()` would be 
^^^ extra whitespace at EOL, please fix

[kolyshkin]

Hope this check is not too restrictive:

• it only works for text files;
• our Go, C, and shell code is already autoformatted, so this is mostly about docs and CI configs.

[kolyshkin]

@AkihiroSuda @thaJeztah PTAL