Rework docs

[camh-]

The README is a bit too detailed for a README. Move most of the content to docs/jsonnet.md and add content for metadata, which is currently implemented but undocumented. Perhaps some of the docs belong in a more generic eval.md document, since there is stuff common to all evaluators (the interface).

The README should focus on getting started and showing simple uses.

[juliaogris]

Not necessarily related to this issue - but maybe worth considering while reworking docs:

Just like we increasingly seem to be refining the Makefile (template) it might be worth while to get some docs template up as well.

In particular:

• README badges
• hermit snippet (what it is, activation, link)
• make help
• contribution section ?

WRT to docs in the docs folder - maybe, very maybe, we should consider to also create GitHub pages for them by default at https://jig.foxygoat.dev or https://jig.docs.foxygo.at ... I'm only suggesting this because sometimes pure markdown falls short, especially wrt syntax colouring of lesser known languages (WSN was one I was looking for). However, it seem jsonnet is supported.