[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

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• 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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Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩ ↩²

[gemini-code-assist]

Code Review

This pull request updates the mustache_template package to version 2.0.4, introducing a new example package and migrating the README.md to use code excerpts. Feedback includes improving the clarity of the changelog entry and adhering to idiomatic Dart lints in the example code by removing unnecessary lint ignores.