Clarify creating plugin SVN tags from trunk

[ekamran]

Summary

Clarifies the "Create tags from trunk" guidance in the Plugin Handbook's Using Subversion page.

Changes

• Removes guidance that suggests checking out the repository root locally.
• Recommends working from a local checkout of trunk/.
• Adds a remote svn cp example for creating a release tag from trunk.
• Adds svn sw examples for switching between a tag and trunk when inspection is needed.

This follows up WordPress/Documentation-Issue-Tracker#78.

Note: #17 also touches this file, but it adds a separate troubleshooting section and does not address this tag-creation guidance.

[ekamran]

@frantorres could you take a look at this one when you get a chance?

This implements Documentation-Issue-Tracker#78, which the docs team already marked [Status] Review. The issue reports that the current "checkout the root of your repository" advice is causing very large local SVN folders, and it suggests working from trunk and creating tags remotely instead. I have used the exact commands from that issue.

No conflict with #17, that one adds a separate troubleshooting section further down the page.