Define more roles for SIG network

Taking some inspiration from https://github.com/kubernetes/community/issues/3076

I think some well-defined roles will help increase contribution and provide opportunities for recognition. Starting this as a place to discuss which roles make sense for the SIG.

These can be regularly rotating positions (e.g., each release cycle, or something similar) to encourage more folks to get involved! As suggested by @parispittman, it would be awesome if each role also had an "understudy" or partner to help spread knowledge.

Here are a few roles I think would be great to have as a starting point for discussion. Obviously they'll need a bit more definition!

### New contributor / community ambassador
Responsible for helping to welcome new contributors to the SIG, representing the interests of new contributors and the rest of the Kubernetes community (maybe two distinct roles?)

### Triage captain
Responsible for organizing the triage of new bugs and issues related to SIG network, maintaining issue backlog.

### Test captain
Responsible for health of automated tests, and SIG network test coverage.

### Project manager
Responsible for helping to define project road map for SIG network.

### Docs captain
Responsible for health of SIG network docs

Start by reading the referenced community issue 3076 and reviewing the proposed SIG network roles in this issue. Clarify responsibilities, rotation cycles, and understudy arrangements for each role; done means the SIG has agreed on well-defined roles.

Written by the indexing model from the issue text.