Infra: Create a Markdown style guide

[sleevi]

To help assist members and CWGs, we should look at documenting good practices for writing Forum Guidelines in Markdown.

The most obvious issue that would benefit from consistency is the numbering style of list items, but other elements, such as section headers, using code format for ASN.1 notation, and how to express tables in a useful way would benefit from documentation, especially for members new to Markdown, but possibly familiar with our Wiki syntax or other such syntaxes.

[srdavidson]

As we now have multiple standards, edited by different people over varying spans, a basic CABF style guide would help us both in good markdown practices but also in maintaining consistency of terminology. Examples might include to harmonise the use of MUST vs SHALL, document the appropriate capitalisation of Definitions.