Add "blockquote" to alerts description to improve SEO

[felicitymay]

Why:

Closes: #27902

What's being changed (if available, include any code snippets, screenshots, or gifs):

The description of the "alerts" syntax now mentions "blockquote" which should make it easier for users to find the information if they search for it after reading the discussion post that introduced the feature: https://github.com/orgs/community/discussions/16925.

Note, we won't be able to confirm the improvement in SEO until after this PR is merged, so the changes should be reviewed on their own merit.

Check off the following:

• I have reviewed my changes in staging, available via the View deployment link in this PR's timeline.

  • For content changes, you will also see an automatically generated comment with links directly to pages you've modified. The comment won't appear if your PR only edits files in the data directory.

• For content changes, I have completed the self-review checklist.

[github-actions]

Automatically generated comment ℹ️

This comment is automatically generated and will be overwritten every time changes are committed to this branch.

The table contains an overview of files in the content directory that have been changed in this pull request. It's provided to make it easy to review your changes on the staging site. Please note that changes to the data directory will not show up in this table.

---

Content directory changes

You may find it useful to copy this table into the pull request summary. There you can edit it to share links to important articles or changes and to give a high-level overview of how the changes in your pull request support the overall goals of the pull request.

Source	Preview	Production	What Changed
get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax.md	fpt ghec ghes@ 3.10 3.9 3.8 3.7 3.6 ghae	fpt ghec ghes@ 3.10 3.9 3.8 3.7 3.6 ghae

---

fpt: Free, Pro, Team
ghec: GitHub Enterprise Cloud
ghes: GitHub Enterprise Server
ghae: GitHub AE

[subatoi]

LGTM: just a minor thought

[felicitymay]

Thanks for the review suggestions @subatoi and the 👍🏻 @jsoref

I think we can merge this, but first I'll need another approval from you Ben, now that I've committed your suggested changes.

[subatoi]

🚀

[github-actions]

Thanks very much for contributing! Your pull request has been merged 🎉 You should see your changes appear on the site in approximately 24 hours. If you're looking for your next contribution, check out our help wanted issues ⚡