Flag auto-generated markup with a new attribute

[stpeter]

On 2021-03-29 (see https://codimd.ietf.org/cmt-20210329# for notes) the XML change management team discussed how to handle markup (e.g., a table of contents or index) that is auto-generated by a tool. The tentative conclusion was to flag such markup using a new attribute so that it's clear where the markup came from.

[miekg]

Note fwiw that mmark adds:

<!-- name="GENERATOR" content="github.com/mmarkdown/mmark Mmark Markdown Processor - mmark.miek.nl" -->

at the top of the XML file