Organize docs by major version, not minor

[chapulina]

Currently, library docs are organized by their minor version. For example, this URL is specific to version 6.4, not to version 6:

https://ignitionrobotics.org/api/math/6.4/index.html

Since we don't break behaviour within a major version, and we often make minor releases, I think it would be more convenient to just separate documentation according to major version, instead of minor. If needed, a tutorial can mention from which minor version the feature is available. This way, in the case above, when we release ign-math 6.5, we don't need to release a whole new set of docs.

[chapulina]

Oh I think this is a duplicate of #47

[chapulina]

Reopening because I think this one is more specific about not using minor versions for API docs.

[chapulina]

That's been fixed! Examples:

• https://gazebosim.org/api/gui/6/classignition_1_1gui_1_1events_1_1LeftClickToScene.html
• https://gazebosim.org/api/math/6/