docs: using multiple repos with tilt

[lianmakesthings]

Currently, the docs on Structuring Multiple Repos is very basic.
It would be good to improve them by e.g. showing an example.

Other questions that could/should be answered in those docs:

How do you deal with merging configs?

• when you load multiple Tiltfiles via load() what happens to variables that have been defined in both Tiltfiles?

Do you need to wrap top level function calls into a method?

[nicks]

this is now updated! #1065