docs: document slate heading as anchor links in user-manual

[nileshgulia1]

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• I signed and returned the Plone Contributor Agreement, and received and accepted an invitation to join a team in the Plone GitHub organization.
• I verified there aren't other open pull requests for the same change.
• I followed the guidelines in Contributing to Volto.
• I succesfully ran code linting checks on my changes locally.
• I succesfully ran unit tests on my changes locally.
• I succesfully ran acceptance tests on my changes locally.
• If needed, I added new tests for my changes.
• If needed, I added documentation for my changes, either in the Storybook or narrative documentation.
• I included a change log entry in my commits.

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If your pull request closes an open issue, include the exact text below, immediately followed by the issue number. When your pull request gets merged, then that issue will close automatically.

Closes #5755

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📚 Documentation preview 📚: https://volto--6750.org.readthedocs.build/

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[stevepiercy]

Thank you! I'm going to merge these suggestions, and see how it looks. I added the link icon into the text.

[stevepiercy]

Here it is! https://volto--6750.org.readthedocs.build/user-manual/blocks.html#copy-slate-headings-as-anchor-links

[davisagli]

Plone editors probably don't know what "Slate" is; it's an implementation detail. I would just remove that word here: "Copy headings as anchor links"