Getting Started with P5 ODDs needs help

We need to work on cleaning up many things about https://tei-c.org/guidelines/customization/getting-started-with-p5-odds/ , and also add information about ideas for and available means of Sanity Checking an ODD customization, connected with https://github.com/TEIC/TEI/issues/1991 . Include info about "TEI for writing TEI Customizations" option available in oXygen. 