Move `Contributing` section and contribution guide into the same place

[jo-mueller]

Currently, there are two places where contributions are explained:

• /contributing/index.md
• /CONTRIBUTING.md

I think these should be in the same place. For me this is relevant regarding #399, but I think it also makes the place tidier aside from that.

[lubianat]

hmm, interesting. /CONTRIBUTING.md is the expected place for this to live on the repo, but does not get built in the website.

Having a single source of truth is probably a good idea. The content of both could be merged.

2 options I see:

1. We find a way to make /contributing/index.md import the content of CONTRIBUTING.md

2. We simply change the GitHub CONTRIBUTING.md file to say that "For the contribution guidelines to NGFF documentation or to the specifications, please read https://ngff.openmicroscopy.org/contributing/index.html."

I am in favor of 02, but whatever works.

Info wrt #276 should be there too, but that may be separate from this issue

pinging @govekk who might have thoughts on this.

[jo-mueller]

Optional 3: Move ./CONTRIBUTING.md into the same folder as ./contributing/index.md, and make it part of the page's toctree. In the readme.md, you can still link to the correct place.

[lubianat]

also works

[jo-mueller]

Option 4: Merge contents of both markdown files into a single document :) (see #410)