Update the style guide to outline how namespaces should be used

## Desired behavior
We currently do not have any rules/guidelines on how namespaces should be used. This could result in naming collisions between various projects. We should outline dos and don'ts of namespace usage for ignition projects.

## Alternatives considered
N/A (new feature)

## Implementation suggestion
Add a section to https://ignitionrobotics.org/docs/all/contributing#style-guides that outlines how namespaces should be included/used. Some guidelines were discussed in this thread: https://github.com/ignitionrobotics/ign-sensors/pull/177#discussion_r774794118

## Additional context
The original conversation for this was started in https://github.com/ignitionrobotics/ign-sensors/pull/177#discussion_r774794118