Enhance service documentation

[savetheclocktower]

This PR does two things.

First: the discussion here makes it clear that the existing documentation on services is confusing. If I knew nothing about them and was trying to make two packages talk to one another, I would read that page and probably make the same errors that the user in that thread was making.

Second: #22 makes a lot of changes and I'm on the fence about most of them. But I figured it was a good idea to remove the least accurate information: the idea that you'll be prompted for your GitHub username and password the first time you publish. The authentication flow works differently now; I'd be tempted to include more information about it, but we do a very good job of describing the authentication process in ppm when it's actually needed, so I figure it's better not to be too specific about it so that we don't have to update it in the future if we tweak the authentication flow. (If someone thinks this should go in its own PR, that's fine.)

This PR also led me to reflect on a pain point I've been hitting: pulsar#1343 describes it well. I think I've got a handle on what the fix would look like and it's not as painful as I thought.

So I'm glad I was asked to explain services this morning — it led to better documentation and also to an eventual bug fix.

[savetheclocktower]

Ping! I might just land this eventually if nobody objects, but it would be good to get some feedback.

[savetheclocktower]

This PR also now contains an overhaul of the “Publishing” section. I was silly to try to address #22 as part of this PR, but at this point it's easier to add another commit to this PR instead of trying to create a new branch that borrows the relevant changes from this PR and removes the rest.

[confused-Techie]

Overall the service documentation is fantastic and really goes in depth in all the right places.
As for the package publishing docs I do like what you've done with it, but wonder if while we are here we should make any notes about needing to register an account with Pulsar prior to publishing? Unless that's elsewhere and I'm just not seeing it in the diff.

Overall these docs are great though