[anoadragon453]

mdbook build was emitting the following warnings:

$ mdbook build
 INFO Running the html backend
 WARN unclosed HTML tag `<version>` found in `usage/configuration/config_documentation.md` while exiting Paragraph
HTML tags must be closed before exiting a markdown element.
 WARN unclosed HTML tag `<details>` found in `openid.md` while exiting Item
HTML tags must be closed before exiting a markdown element.
 WARN unexpected HTML end tag `</details>` found in `openid.md`
Check that the HTML tags are properly balanced.
 WARN unexpected HTML end tag `</table>` found in `development/synapse_architecture/cancellation.md`
Check that the HTML tags are properly balanced.
 WARN unclosed HTML tag `<tr>` found in `development/synapse_architecture/cancellation.md`
 WARN unclosed HTML tag `<table>` found in `development/synapse_architecture/cancellation.md`
 INFO HTML book written to `/home/runner/work/synapse/synapse/book`

which made reading the output cumbersome. This PR fixes them up.

I briefly thought about promoting these warnings to errors, but mdbook does not offer such an option yet.

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Code style is correct (run the linters)