Document all injected environment variables on checker pods

[integrii]

We should add KH_CHECK_RUN_DEADLINE to this doc along with all the other environment variables that are loaded into pods by KH by default.

[bavarianbidi]

have to rework on PR #443 and could add this in my PR if you want?

[integrii]

Sure! Up to you. I tend not to keep adding into PRs because things go wrong and the extraneous changes get dropped by accident. More smaller PRs is normally better practice, I think.

Either way is fine here, though.

[jonnydawg]

All the crucial ones are documented (all the ones check implementers should be aware about)