Update documentation/other docs, enforce, SPDX license

Rough list of tasks

• split (and update) README - BUILDING, CONTRIBUTING, other
• CONTRIBUTING - pick any bits from ^^, fold docs/code-style.txt, document clang-format, document better sob (add in-tree copy of cert?)
• Add ci-fairy hooks for access, sob, other - see libinput for inspiration
• Document the use of SPDX, update the whole project
• manuals lack SPDX license identifier, re-license, rewrite?