docs: Add long click listener to cluster demo

[barbeau]

Follow up to implementing the feature in https://github.com/googlemaps/android-maps-utils/pull/752.

Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• [x] Make sure to open a GitHub issue as a bug/feature request before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• [x] Ensure the tests and linter pass
• [x] Code coverage does not decrease (if any source code was changed)
• [x] Appropriate docs were updated (if necessary)
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[codecov[bot]]

Codecov Report

Merging #754 (e4d4592) into main (7a9c747) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #754   +/-   ##
=======================================
  Coverage   39.20%   39.20%           
=======================================
  Files          71       71           
  Lines        4076     4076           
  Branches      609      609           
=======================================
  Hits         1598     1598           
  Misses       2375     2375           
  Partials      103      103           

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[googlemaps-bot]

:tada: This PR is included in version 3.1.0 :tada:

The release is available on:

• v3.1.0
• GitHub release

Your semantic-release bot :package::rocket: