Update the style guide to outline how namespaces should be used

[adlarkin]

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: gazebosim/gz-sensors#177 (comment)

Additional context

The original conversation for this was started in gazebosim/gz-sensors#177 (comment)

[mjcarroll]

I think it would be helpful to generally follow the cppcore guidelines, specifically SF 6 & 7

https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#sf6-use-using-namespace-directives-for-transition-for-foundation-libraries-such-as-std-or-within-a-local-scope-only

https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#sf7-dont-write-using-namespace-at-global-scope-in-a-header-file

[scpeters]

I think it would be helpful to generally follow the cppcore guidelines, specifically SF 6 & 7

https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#sf6-use-using-namespace-directives-for-transition-for-foundation-libraries-such-as-std-or-within-a-local-scope-only

https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#sf7-dont-write-using-namespace-at-global-scope-in-a-header-file

that makes sense, both avoiding using namespace statements at global scope in a header file and preferring local scope when possible

I don't know if there are any linters that can detect this

[mjcarroll]

clang-tidy can get it: https://clang.llvm.org/extra/clang-tidy/checks/google-build-using-namespace.html