Support separate, accessible labels for Nimble components

😯 Problem to Solve

There are use cases where a client may wish to not use a control's built-in label (if exists), but rather provide a separate label element for the control. This is currently the only way to label a select or combobox. In such use cases, we still want the control to have an accessible name. There should be some clear pattern (that we can document) for achieving this, e.g. through the use of aria-label, aria-labelledby, or <label for="...">.

💁 Proposed Solution

We should look at each of our controls and ensure there is some way to give the control an accessible name, even when not using the standard, built-in label. We should try to use the same pattern for all controls. Whatever approach we come up with should be documented in nimble-components\docs\accessibility.md.

Start by reviewing the controls and nimble-components\docs\accessibility.md to identify the existing labeling patterns and the controls that need a separate accessible name. Define a consistent approach that works across the controls, verify each control supports it, and document the usage pattern in accessibility.md; no tests or specific control files are named.