Integrate STYLEGUIDE.md into the main documentation site

[st-pasha]

What could be improved

The STYLEGUIDE.md should be merged into the main documentation site, for better visibility. In addition, the guide needs to be substantially cleaned up as it contains content that is either outdated, or enforced via lint rules. At the same time, it can be expanded to included some new conventions that we have introduced over time.

Currently that guide is not linked from anywhere, so it is unlikely anybody reads that document.

Any risks?

None.