docs: add gitlab example to check all commits in merge request

[bliuchak]

Description

Add an example for Gitlab CI/CD that shows how to check all commits in merge request.

Also solves this question on Stackoverflow.

Motivation and Context

Current CI/CD guide shows examples how to validate single commit only in Gitlab CI/CD.

Usage examples

// commitlint.config.js
module.exports = {};

echo "your commit message here" | commitlint # fails/passes

How Has This Been Tested?

I've tested this with -V on branch with multiple commits (including one non-valid commit).

Positive (multiple valid commits):

// Gitlab CI/CD runner
// ...
$ npx commitlint -V --from $CI_MERGE_REQUEST_DIFF_BASE_SHA --to $CI_COMMIT_SHA
⧗   input: chore: good
✔   found 0 problems, 0 warnings
⧗   input: chore: bad
✔   found 0 problems, 0 warnings
⧗   input: feat(cicd): run commitlint in pipeline
✔   found 0 problems, 0 warnings
// ...

Negative (multiple commits including one invalid):

// Gitlab CI/CD runner
// ...
$ npx commitlint -V --from $CI_MERGE_REQUEST_DIFF_BASE_SHA --to $CI_COMMIT_SHA
⧗   input: chore: good
✔   found 0 problems, 0 warnings
⧗   input: bad
✖   subject may not be empty [subject-empty]
✖   type may not be empty [type-empty]
✖   found 2 problems, 0 warnings
ⓘ   Get help: https://github.com/conventional-changelog/commitlint/#what-is-commitlint
⧗   input: feat(cicd): run commitlint in pipeline
✔   found 0 problems, 0 warnings
// ...

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [x] Chore (documentation update)

Checklist:

• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.

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[escapedcat]

@knocte wdyt?

[knocte]

I haven't used GitLab in ages. Hey @bliuchak, shouldn't this test first that the variable CI_MERGE_REQUEST_DIFF_BASE_SHA contains a value? (Because, if it doesn't, then the CI job is being spawned by a push, not a mergeRequest, and then in that case we wouldn't want to fail?)