Add blockquote to Basic writing and formatting syntax

[jsoref]

Code of Conduct

• I have read and agree to the GitHub Docs project's Code of Conduct

What article on docs.github.com is affected?

https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#alerts

What changes are you suggesting?

1. Add blockquote to the article
2. Ensure that searching for note+warning+blockquote+markdown finds the article (and not on page 9/10).

Additional information

In https://github.com/orgs/community/discussions/16925#discussioncomment-6876291, I wrote:

there's no sign of documentation for this feature

The feature announcement https://github.com/orgs/community/discussions/16925 is titled [Markdown] An option to highlight a "Note" and "Warning" using blockquote

It's reasonable to expect a search for note+warning+blockquote+markdown to turn it up. Indeed, 3/4 of those search terms are present on https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax -- only blockquote is missing.

[cmwilson21]

@jsoref Thanks for opening an issue! I'll get it triaged 👍

[felicitymay]

Thanks for flagging this @jsoref ✨

Your suggestion makes a lot of sense. I'll update the issue summary with a brief content design plan and label the issue for the upcoming Hacktoberfest.

[felicitymay]

On reflection, I opened a PR because we already have a help-wanted issue for this section of the article and it would be good to avoid OS contributors have conflicts in their work.

• Add "blockquote" to alerts description to improve SEO #28140