"Timeline Structure" document elides rates from its diagrams but doesn't note that

In the diagrams in the [Timeline structure document](https://opentimelineio.readthedocs.io/en/stable/tutorials/otio-timeline-structure.html), ranges are represented like:

`3, 3`

This means `[3/X, 3/X)` where X is the rate of the media, or a Range that starts at time 3/X seconds and has a duration of 3/X seconds.

We should clarify that or add some text to describe what we mean. Explicitly noting that nothing enforces a rate systemically (IE tracks don't have a consistent rate relative to each other, nor do time ranges necessarily) would be a good idea. Additionally, a note about how, or if, rates are enforced or quantized is a problem left to the integrator.

Page can be seen here: https://opentimelineio.readthedocs.io/en/stable/tutorials/otio-timeline-structure.html

Open the linked Timeline structure document and inspect the diagrams that use ranges such as `3, 3`. Clarify that the values are rate-relative, explain that rates are not systemically enforced or necessarily consistent, and note that integrators determine rate enforcement or quantization; done means the rendered page explains these semantics clearly.

Written by the indexing model from the issue text.