Output count of linted docs in vale

What does this MR do and why?

The vale linter does not output the count of linted files.

We removed the count in !78431 (merged) because it only reported the number of files with issues, which was confusing when it said 0 files.

But I think we all completely forgot that @ashmckenzie enabled this for us with https://github.com/errata-ai/vale/pull/270. We never did the last step of making use of that in the template.

This does seem to work well locally, but not with globs. So instead let's use doc/ instead of a glob when we need to lint all files. In the latest test, we can see it works (https://gitlab.com/gitlab-org/gitlab/-/jobs/7198515753):

INFO: Looking for Vale to lint prose, either installed locally or available in documentation linting image...
INFO: Found locally-installed vale! Running...
0 errors, 0 warnings, and 0 suggestions found in 2290 files.

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