Simplify the contribution guidelines

The current contribution guideline is too wordy to contributors, old and new. As a reviewer myself, I have a difficult time to find out a guideline when attempting to quote it or refer a contributor to it. For example,

- We are not accepting "dual-hosted" or "3rd party" contents, where are those guide?
- We don't fix blog pages that are over 12 months old, where exactly is this mentioned?

If I just started reading the contribution guides, there are so many pages. The left hand navigator has no logic in it. The fonts are almost the same size. We have for "writing new topic" and "contributing new content", we have "reviewing changes" and "participating in SIG Docs", we have "Custom Hugo Shortcodes" and "advanced contributions", "content guide" listed alongside "content organization" and "page content types" ...

Let's consider how to tailor and reorganize this. Contributing to k8s docs should not be so difficult as it smells today.

Start by reviewing the contribution-guide pages, the left-hand navigator, and the sections named in the issue, including content guidance, organization, page content types, and Hugo shortcodes. Done means the contribution guidance is reorganized into a clearer, less verbose structure and explicitly covers the rules cited in the issue.

Written by the indexing model from the issue text.