[Beats docs] Update contributor guidelines and (broken) links to the guidelines

[kilfoyle]

Updates are needed in the Beats docs for the documentation section of the "Contribute to Beats" page and to Contributing to the docs.

• These sections refer to Asciidoc format and to source file locations that may no longer apply.
• More importantly, it's not yet clear what to do with the Generated docs section, since the generation scripts should be either converted to generated markdown or removed so that the files are maintained by hand. For that discussion I've opened: Update Beats code to support new Markdown documentation beats#43008
• As well, there are links to the "Beats Developer Guide" that are broken, for example here and here.

Notes:

• Marking as high priority since this is already affecting contributers.
• I've tentatively added this to Sprint 33 but there's a large dependency on issue 43008 above.

[colleenmcginnis]

@kilfoyle are you still working on this?

[kilfoyle]

@colleenmcginnis Nope, not at all. I opened it just so we don't lose track of the work. I've unassigned myself. :-)

[bmorelli25]

@colleenmcginnis -- closed in elastic/beats#46107?

[colleenmcginnis]

I think so!