[DevHub] Improve best practices in our Using Subversion guide

Issue Description

Hi!

I submitted a new plugin a few days ago and got SVN access to the plugins repository, as well as a congratulations e-mail, with a link to the Using Subversion document. The documentation mentions things like "Checkout the root of your repository" which is exactly what users should not be doing, and likely the cause of many complaints in the WordPress community about their Subversion folders being over 80GB in size.

Instead, they should checkout the trunk directory of their repository and commit changes there. If they need to tag a release, this can (and should) be done remotely:

svn cp ^/plugin-slug/trunk ^/plugin-slug/tags/1.0

If one needs to switch to a tag it's:

svn sw ^/plugin-slug/tags/1.0

Then back to trunk:

svn sw ^/plugin-slug/trunk

To delete an old tag

svn rm ^/plugin-slug/tags/0.9

URL of the Page with the Issue

https://developer.wordpress.org/plugins/wordpress-org/how-to-use-subversion/

Start with the Using Subversion guide at https://developer.wordpress.org/plugins/wordpress-org/how-to-use-subversion/ and review its checkout instructions. Update the guidance to use the trunk directory, include the remote tag, switch, and delete commands shown in the issue, and remove advice to check out the repository root. Done means the guide reflects these best practices clearly.