Add description of measurement tools

[dennis-tra]

Changelog:

Added description for:

• Nebula
• Parsec
• Tiros

Copied a bit from the repo's readmes but also created a bunch of new text

[yiannisbot]

The issue of being able to keep things up-to-date without having to modify things in multiple places is important here. How about adding a sentence in the end (e.g., under "Contributing") to say that "The source of truth and latest updates are in the tool's repository. This description might fall out of date and therefore the reader is encouraged to check the individual repo for details.", or something similar? That's not to say that we should not make an effort to keep things updated here, but more as a disclaimer for those that are looking into the details.

[iand]

@dennis-tra is this ready for a second look?

[dennis-tra]

Thanks everyone for the reviews. I revised the pages quite significantly. I aimed to provide rather timeless information linking to the relevant source code parts where appropriate. I focussed more on concepts and our deployment.

I updated parsec to reflect our latest IPNI addition and extended the description of Nebula.