docs: document the DORA source branch pattern for lead time for changes

[chenwei791129]

Summary

Companion to apache/devlake#9229, which adds a DORA project setting, Source branch pattern (prHeadRefPattern). When it is set, only merged pull requests whose source branch matches the regex count toward Lead Time for Changes. This PR is a draft until apache/devlake#9229 is merged, since the option may still change in review.

File	Change
docs/Metrics/LeadTimeForChanges.md	New section Choose which pull requests count (optional): why to use it, where to set it, how the pattern is matched (field, RE2 syntax, anchors, case, refs/heads/ prefix, empty and invalid values), examples, which metrics it affects, and the equivalent dora metric plugin option. A one-line pointer to it at the end of "How is it calculated?"
docs/DORA.md	Short Count only some pull requests section linking to the new section, next to "DORA for a monorepo"
docs/Metrics/PRCycleTime.md	One sentence noting that only matching PRs are measured when the pattern is set

Does this close any open issues?

Related: apache/devlake#9222 (closed by apache/devlake#9229).

Screenshots

N/A. Text only.

Other Information

• The version note says "available after the v1.0.3-beta18 release". I will update it if a new release ships before feat(dora): filter pull requests by source branch pattern in lead time for changes devlake#9229 is merged.
• The "Invalid expression" row describes the current behavior: the blueprint fails to start. If fix(project): validate metric plugin options when saving a project devlake#9228 (validate metric plugin options on save) is merged first, I will change it to say the project settings cannot be saved.

Verification: npm run build passes for both locales with onBrokenLinks: 'throw', and I checked the three pages with npm run serve.