Adapt docs for new KEPTN_CONTEXT env variables

[mowies]

Goal 🥅 ⚽️

Adapt the docs according to the mentioned issues for deno and python runtime.
Add new docs for the context variable that's injected into the container runtime.

Feature PR: #2491

DoD

• Update deno runtime readme
• Update python runtime readme
• Make sure that all occurrences of the CONTEXT variable are fixed in the documentation
• Make sure all links are still working

[StackScribe]

What should be the relationship of these README files to the docs at http://localhost:1314/docs/guides/tasks/#context? Right now, it seems that each has part (but not all) of the information a user needs and the two do not link to each other. I would favor putting everything in the docs and linking to the docs from the README but we need to decide this as a group. And we can merge this PR then I'll do a separate PR if want the info in the docs.

[mowies]

The readme can have more developer focussed info, but the docs should still be complete I would say