Add an introduction to the 2D index page

[tetrapod00]

Addresses #4108.

A bit of a stub introduction, but serves its purpose of summarizing and providing links to related sections.

Note that we recently added a more colorful introduction to 2D in #9621. It's still an open question what the ideal structure for introducing sections of the manual is. Some sections include multiple paragraphs on the index, some sections include a short intro on the index and a separate long introduction page, and most sections still only include links on the index page.

[mhilbrunner]

Thanks!