writing-style-guide: add headings style guidance

[mschristensen]

Description

Updates the writing style guide to provide guidance on using the imperative form in headings.

See discussion:

• https://ably-real-time.slack.com/archives/C01LR5XFHFX/p1767628669595849
• https://ably-real-time.slack.com/archives/C01LR5XFHFX/p1767628739770889

Checklist

• Commits have been rebased.
• Linting has been run against the changed file(s).
• The PR adheres to the writing style guide and contribution guide.

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