Add docs for the UI

## Requirement - what kind of business use case are you trying to solve?

The UI is not documented anywhere. (Aside from the configuration options.)

## Problem - what in Jaeger blocks you from solving the requirement?

Currently, the UI code is the only place that will inform users how the UI works, aside from the UI configurtaion.

For instance, if a user wanted to understand the search in the trace detail page, checking the code or PR is the only way they would learn it's case-insensitive and prefacing a term with a hyphen, e.g. `-stacktrace`, will exclude keys containing the term from the tags or log fields checked.

## Proposal - what do you suggest to solve the problem or improve the existing situation?

Create a top-level section in the docs for the UI. The reason I suggest a top-level section is because it doesn't seem to fit within the existing sections.

## Any open questions to address

Mainly if this is a good idea or not.

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cc @aljesusg who is preparing docs for a new feature.

Start by reviewing the existing documentation structure and the UI behavior described in the issue, including trace-detail search. Define the scope of a top-level UI section and document the cited case-insensitive search and hyphen-exclusion behavior; done means users can learn how the UI works without reading source code.

Written by the indexing model from the issue text.