Rework contributing pages so that they are DRY

[chalin]

We have contributing guidance in two places (soon to be three):

• https://www.docsy.dev/docs/contribution-guidelines/
• https://github.com/google/docsy/blob/main/CONTRIBUTING.md

We should consolidate the information, and have one canonical source. FYI, we went through a similar exercise in OTel:

• Contributing guidelines should be consolidated in one place open-telemetry/opentelemetry.io#4652

[chalin]

Fixed by removing dup content, and cross-linking pages:

• Set version to 0.14.2-dev-001-over-main-20ff86e7, and more #2542
• Update blog and CL for 0.14.2, refactor contrib pages #2550