Add content to READMEs

[Frando]

The READMEs in the repo (root and individual iroh crates) are mostly empty. The READMEs are displayed on crates.io and possibly other places once the crates are published, and also are usually the first entry point for people browsing the Github repo.

Would be great to add high-level descriptions of the crates and the repo there.

• root docs: update root, iroh, iroh-metrics readmes #1258
• iroh docs: update root, iroh, iroh-metrics readmes #1258
• iroh-bytes @rklaehn docs(bytes): add more about text to README.md #1255
• iroh-net @flub docs: Fill in the iroh-net README a little #1252
• iroh-metrics docs(bytes): add more about text to README.md #1255