Port changes from 7x Docs for restructure of editor-plugin-version.adoc

[FarzadHayat]

Ticket: DOC-2450

Site: Staging branch

HOTFIX: same changes as hotfix/7/DOC-2450 but for 6 Docs

Changes:

• Add examples of specifying minor and patch versions
• Move the "Difference between using TinyMCE 6, 6-Testing, and 6-Dev release channels" section to the top of the document so that information about specifying editor version is all in one place.

Pre-checks:

• Branch prefixed with feature/<version>/, hotfix/<version>/, staging/<version>/, or release/<version>/
• [-] modules/ROOT/nav.adoc has been updated (if applicable)
• [-] Files has been included where required (if applicable)
• [-] Files removed have been deleted, not just excluded from the build (if applicable)
• [-] Files added for New product features, and included a release note entry.

Review:

• Documentation Team Lead has reviewed