running.md moved to wasmtime.md section and updated w custom invocations

[macovedj]

closes #264, also as part of updates to chapter 6 discussed on SIG Docs meeting.

[itowlson]

Does mdbook give us a way to keep a redirect here, just in case?

Also, I again have a bit of a concern re signposting. The "creating and consuming" section provides a kind of journey of I write my component and I compose it with other components and then I run it and then I publish it. And removing this leaves a gap. The reader now needs to know that "how do I run my component" lives in the "Runtime Support" section, the title of which feels like it's going to be a list of caveats and feature comparisons for different runtimes rather than "this is how you make your component go."