Add server-side library development section, link to log levels; improve wording

[ktoso]

resolves https://github.com/swift-server/guides/issues/38

[kmahar]

I like the suggestion to sort the items in the list into categories a little more, especially given that we want to keep building out guides further and expect this list to grow. we could probably group all the debugging related links under one "debugging" top level bullet?

[ktoso]

Sounds good, lemme try to do that here

[ktoso]

What about this @kmahar @tomerd ?

I also applied some of nice @wsargent's comments from the other thread

[ktoso]

Thanks!