feat: Add new PageHeader.LeadingElement component

[kurtdoherty]

Context

• After PageHeader was implemented in v5 component refactor: PageHeader #490, Design made some updates regarding the leading element which require us to introduce a new PageHeader.LeadingElement.

This PR

• Introduces a new PageHeader.LeadingElement subcomponent that helps size the leading element correctly based on its type: icon vs image.
• Scouts some broken links in the change log.

[rpt-uk-github]

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[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
Report missing for d7caa7a1	✅ 100.00%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (d7caa7a)	Report Missing	Report Missing	Report Missing
Head commit (c3fc788)	7457	6737	90.34%

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#607)	12	12	100.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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Footnotes

1. Codacy didn't receive coverage data for the commit, or there was an error processing the received data. Check your integration for errors and validate that your coverage setup is correct. ↩