Update contribution resources to account for Objects

Update contributor guidelines, MR templates, component lifecycle, and others…

1. !2726 (merged) and !2744 (merged) — Update MR template to be generic for any kind of documentation update, including objects, and update its links.
2. !2727 (merged) — Expand on object overview page, moving generic content out from the individual object pages.
3. Figma branch (merged) — Add example page and legend to conceptual model file with stickies explaining components (object and lines). Add legend component to each object page in Figma.
4. !2769 (merged) — Embed conceptual model example page in the objects overview page.