Reduce duplication of plugin publishing guides

There's an obvious duplication between help pages at https://plugins.gradle.org/docs/submit and https://plugins.gradle.org/docs/publish-plugin vs more throughout and step-by-step guide at https://guides.gradle.org/publishing-plugins-to-gradle-plugin-portal/. We could point the documentation from the plugin portal to the appropriate guide to avoid it.

@etiennestuder @ldaley Any interest from your side to make such change at this point in time? Even though it's a minor, someone familiar with the Plugin Portal from your team would have to be involved. @ysb33r wrote this guide and could make changes in it with needed, so from that perspective it's a good timing. After July we will be on our own.

Compare the Plugin Portal pages at plugins.gradle.org/docs/submit and /docs/publish-plugin with the guide at guides.gradle.org/publishing-plugins-to-gradle-plugin-portal/. Start by identifying duplicated publishing instructions and determine where the portal pages should link to the guide. Done means the duplicated guidance is removed or reduced and readers are directed to the appropriate step-by-step guide.

Written by the indexing model from the issue text.