Updates to contributors guide

[alexakreizinger]

got rid of some defunct/redundant sections, played musical chairs with other sections for clarity, and added a "quick tips" section at the top for anyone who's just skimming.

if I went too far removing stuff, I can definitely revert some of these changes, but my thought process there is that too much content makes it harder for contributors to find (and follow) the guidelines that matter most. same rationale there for putting certain items in a collapsible <details> element.

I also opted to remove the list of active vale/markdownlint tests because I think that info is more interesting to maintainers than to contributors, and therefore more trouble than it's worth to keep current. (unless I'm mistaken, some of the warning-versus-error distinctions were already out of date.) again, not a hill I'll die on, but I would argue that linking to the place where the tests actually live is more than enough.

also also, the diff for this one is not super cute. to see how the updated contribution guide actually renders for readers, go here: https://github.com/fluent/fluent-bit-docs/blob/alexakreizinger/sc-143151/updates-to-contributor-s-guide/CONTRIBUTING.md