Rewrite the Contribute page and fix its section links

[Dushyant-web]

This rewrites the Contribute page based on the outline in #202 and the feedback there.

Fixes #202
Fixes #174

What's on the new page:

• A short intro on what OpenPrinting is, plus a "Not sure where to start?" table for newcomers
• Your first steps, for people who have never contributed to open source
• Join the community, Help other users, Report bugs, Contribute code, Test printers and releases
• Documentation and translations, now with CODA (OpenPrinting: Developer documentation for libcupsfilters2 API needed canonical/open-documentation-academy#74)
• Google Summer of Code (GSoD removed), printer manufacturers (only driverless printing and Printer Applications) and donations
• A few terms explained, a short glossary at the end (CUPS, IPP, PPD, fork, pull request, ...)

"Your first steps" and the glossary weren't in the outline. I added them so someone who is completely new can follow the page, but I'm happy to drop them if you prefer.

Every section is a normal heading and the table of contents is turned on, so the sub-links from #174 work now. The old #data, #bugs, #support, #programming and #otherstuff anchors are still there, so existing links don't break.

Only contents/pages/contribute.md is changed.

Testing:

• yarn lint and yarn verify:urls pass
• Checked the page with yarn dev in light and dark mode, on desktop and phone width
• Checked that every link opens and every in-page link reaches its section
• I couldn't run yarn build on macOS because of the Foomatic tools, so I'm relying on CI for that

Screenshots:

cc @prisorue @n0y0nD @izmalk for the content review