[mustache_template] add example app and migrate README to excerpts

[mozammal-hossain]

This PR adds a runnable example/ app for mustache_template and migrates README usage snippets to excerpt-managed code from that example. This addresses the missing example app noted in the issue and aligns the package with the repository’s code-excerpt documentation pattern.

It also removes mustache_template from script/configs/temp_exclude_excerpt.yaml now that excerpt support is in place, and includes the corresponding version/CHANGELOG updates.

Fixes flutter/flutter#183936

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style, or I have commented below to indicate which documented exception this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

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