Add support for kbd, btn and menu macros

[slonopotamus]

Subj, Asciidoctor has very cool macros to render keyboard combos, button labels and menus:

https://docs.asciidoctor.org/asciidoc/latest/macros/ui-macros/
https://docs.asciidoctor.org/asciidoc/latest/macros/keyboard-macro/

Currently:

1. kbd is displayed in Confluence as regular text, while this is how it looks in HTML:
   
2. menu is displayed as bold text without decorations in Confluence, while this is how it looks in HTML (note presence of arrow symbols):
   
3. btn is displayed as bold text without decorations in Confluence, while this is how it looks in HTML (note presence of square brackets):