Export requirements roadmap for test tooling consumption

This continues the roadmap from #402 to generate artefacts from the ASCIIDoc source, the single "source of truth", for external tooling, such as requirements management tools, to consume. A separate PR is planned for each part below.

Part 1: add support to the document processor to extract actors, transactions, content modules, etc was merged in PR#402.

Part 2: Add markup to SDPi source documents:

• profiles & profile options
• use cases
• content modules
• protocols
• actors
• transactions
• requirements
• tweak code generation/markup based on discussion with SDPi editors and how the above goes
• add automatically generated tables using new features
• review/resolve any appearance changes arising from semantic markup.
• incorporate OIDs from #449
• figure out how to deal with placeholder transactions

Part 3: Add external conformance references using the 11073-10700 as an example:

• process xml metadata to artefacts in a useful format
• extend processing (if necessary) to reference external conformance artefacts
• include external conformance reference in SDPi source in a suitable format

Part 4: next steps

• incorporate (any) issues comments into the road map.
• think about capturing actor relationships, particularly inheritance, so we don't need duplicate content in the source.
• standardize / simplify naming of block, macro and attributes.
• remove generated artefacts from repository and add a .gitignore file to exclude generated content.