Docs: Add Vale sentence spacing rule

What does this MR do?

This is a follow-up to !47136 (closed). There, I mentioned another "possibly … useful" reg-ex to find incorrect sentence spacing, which I suggest here. However, it doesn't seem to be very useful after all, because it matches just a few % more or less and different locations, counting the those as follows:

cd gitlab-org/gitlab/
git restore --source=3b90528db32cf3cda1d286344c36d3a409c18948~1 doc/*  # before gitlab-org/gitlab!23928

vale doc/ | grep 'must contain one' > vale-XYZ-rule.txt
# 3 times according to table below with different XYZ
# incomplete message from SentenceSpacing.yml because Vale output got broken into 2 lines, not sure why.

# also 3 times for the different XYZ
vale-XYZ-rule.txt | wc -l

# Cursory inspection of
diff vale-X-rule.txt vale-Y-rule.txt vale-Z-rule.txt

Results

This rule	N hits
not added	208
added as 3rd	207
replacing 1st	200

Because of this low difference, and

1. #199367 (closed) & its follow-up MRs cleaned up the majority of hits already, and
2. seeing the risk of leaving a few typos and such in our docs as actually an opportunity to remain open for "legitimate peripheral participation",

I would not change the Vale rule after all and close this MR. To me, it seems my assumption in !47136 (closed) was prematurely optimistic 😅 I also cancelled the pipeline for this reason.

However, I'm sure you know better testing regimes for the quality and usefulness of such a reg-ex / rule, so please feel free to take over this MR if you see potential.

Related issues

&2497 (closed) & #199367 (closed).

Author's checklist (required)

• [-] Follow the Documentation Guidelines and Style Guide.
• Apply the documentation label, plus:
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  • development guidelines when changing docs under doc/development/*, CONTRIBUTING.md, or README.md.
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• Assign the designated Technical Writer.

When applicable:

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• [-] Link docs to and from the higher-level index page, plus other related docs where helpful.
• [-] Add GitLab's version history note(s).
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• [-] Add/update the feature flag section.
• [-] If you're changing document headings, search doc/*, app/views/*, and ee/app/views/* for old headings replacing with the new ones to avoid broken anchors.

Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

2. Technical Writer

• Optional: Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.
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  • Add docs-only when the only files changed are under doc/*.

3. Maintainer

1. Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
2. Ensure a release milestone is set.
3. If there has not been a technical writer review, create an issue for one using the Doc Review template.