docs(handbook): Refactor the Pull Requests section in the Contributing document

[ppawlowski]

Description

This pull request refactors the Pull Requests section in the Contributing document in the handbook. It focuses primarily on:

• adding a pull request title requirement
• polishing the wording throughout the section: tightening sentences, removing the redundant repetition of "Pull Request(s)" by introducing the "PR" abbreviation on its first use, and merging the two pre-staging paragraphs into one

Related Issue(s)

https://github.com/FlowFuse/engineering/issues/4

Checklist

• I have read the contribution guidelines
• I have considered the performance impact of these changes
• Suitable unit/system level tests have been added and they pass
• Documentation has been updated
• For blog PRs, an Art Request has been created (instructions)

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