Use stronger typing for buffer protocol objects in gl module

[pushfoo]

Closes #1227 and goes beyond the initial ticket. There were multiple places with weak or absent typing worth improving. There's also a new programming guide section that is linked from the docstrings of the more commonly used objects.

Since GL objects have subtle differences in write behavior, there are two union types:

1. BufferProtocol as a top-level arcade type to cover general cases
2. BufferOrBufferProtocol in gl.types for cases where a gl.Buffer is also acceptable.