(docs) Reorganize content for "Docs Contributions"

[tesseralis]

Summary

Reorganize and split up the content of "Docs Contributions" page and related pages for clarity.

Motivation

When I looked through the "Docs Contributions" page, I found it confusing and its content mixed in and repeated with other pages such as "Doc and Website Components".

Steps to resolve this issue

First, create a new "Writing docs in Markdown" page. This file should contain the sections of "Docs Contributions" and "Docs and Website components" that deal with actually writing docs in markdown. These include:

• frontmatter options (e.g. table of contents, overview, stub issues)
• available MDX components
• other formatting options (headings, code blocks)

Second, split up the content of "Blog and Website Contributions" into "Blog Contributions" and "Website Contributions". "Blog Contributions" should contain most of the content of the old page. "Website Contributions" should contain:

• The section "Making changes to the website" from "Blog and Website Contributions"
• The section "Styling components on Gatbyjs.org with Theme-UI" from "Docs and Website Components"