Implement Asciidoctor macros for release notes documents

[joaquintides]

This is connected with the new release notes procedure. The current qbk-based mechanism for release notes production makes available some "macros" defined here:

https://github.com/boostorg/website/blob/master/feed/ext.qbk

Many of these macros are not in actual use in the release notes (meaning I haven't seen them used recently or ever). The ones that I've seen are (see the latest qbk-based release notes and the generated output):

• [phrase library..XX]: produces a header for the section on lib X.
• [@/libs/function/ Function] creates a link to the docs for the library (Function in the example). I understand that
• [github_pr geometry 1247]: produces this: PR#1247
• [github geometry 1231]: produces this: #1231
• [@/tools/build/ Build]: I think this is formally equivalent to [@/libs/function/ Function].

We need to have equivalent Asciidoc macros for the benefit of Boost authors contributing their individual notes in the new release notes procedure.

[daveoconnor]

@sdarwin and @pdimov for this ticket I'll need a new repo e.g. asciidoctor-boost and an account on rubygems for providing a distributable package.

It probably makes more sense that that rubygems account be tied to the c++ alliance/boost than me, and Rob suggested you'd be the people to talk to about that. Can you help?

[sdarwin]

@daveoconnor you are an admin of https://github.com/cppalliance/asciidoctor-boost