feat(doc): add mkdocs-deploy.yml for doc ci

[griff-rees]

feat(doc): initial version without kroki

This follows mkdocs-materials but we may end up needing a full docker version to use kroki (see local.yml for details).

[griff-rees]

@andrewphilipsmith: finally a bit more doc stuff ;). Cool if you'd like to review (no worries if not). This doesn't address the issues below, but it would get some docs up to ease answering questions sooner...

• Automate mkdocs docker local.yml kroki render #116
• Server diagram rendering config only works after a local kroki run and within a docker compose -f local.yml deploy #115