Add `CesiumTileExcluder` tutorial

We get a lot of questions about how to exclude tiles, and always direct users to #248. It'd be great to turn the contents of the PR description into an official tutorial.

Start with issue #248 and read its pull request description, since the requested tutorial should be based on that material. Identify the repository's existing tutorial location and format, then adapt the tile-exclusion explanation into an official CesiumTileExcluder tutorial. Done means users can follow the tutorial instead of being redirected to #248.

Written by the indexing model from the issue text.