[RFE]: Provide full schema docs for presubmit.yml

[jsharpe]

What happened?

Changes requested to presubmit.yml in PR review; schema of file / best practice has changed since last submission of module

Version

N/A

How to reproduce

No response

Any other information?

Docs currently link to CI repo for reference for presubmit.yml but its not really documented there. Docs provide some examples but there is not a full reference of all valid keys / structure that the code can parse.
Workaround tends to be to find a similar module and copy presubmit.yml from them instead of having reference documentation to construct a file from scratch from.

[jsharpe]

Note that https://github.com/bazelbuild/continuous-integration/tree/master/buildkite#configuring-a-pipeline is not a full specification for the format - its documentation by example; it doesn't list for instance the valid sets of keys under each section within the yaml file.

[jsharpe]

@Wyverald I don't think that PR resolves this RFE - it doesn't provide a full schema definition for the yaml file; I want a complete set of valid keys under each section documented. Please reopen this issue.

[Wyverald]

@meteorcloudy