[mustache_template] Add example app and code excerpts

[HibaChamkhi]

Adds an example app for the mustache_template package, adds code excerpts,
and removes the package from script/configs/temp_exclude_excerpt.yaml.

Fixes flutter/flutter#183936

[gemini-code-assist]

Code Review

This pull request introduces a Flutter example application for the mustache_template package and refactors the README.md to utilize code excerpts from the new example. Additionally, the package is removed from the excerpt exclusion list, and the version is bumped to 2.0.5. Feedback suggests making the changelog entry more descriptive regarding the specific features demonstrated by the example app.

[gemini-code-assist]

The changelog entry for the example app is redundant with the description of the change. Consider being more specific about what the example app demonstrates.

[stuartmorgan-g]

Thanks for the contribution!

Please look over the review feedback on the previous attempted PRs that referenced this issue, and make sure that your PR addresses all of that feedback. For instance, this PR significantly rewrites the README in ways that have nothing to do with the issue, which as discussed previously is not in scope for this PR, and this also doesn't have meaningful tests, as was flagged in several previous PRs.

Also, as this package does not have a Flutter dependency, the example app should be a Dart script, not a Flutter app.

[HibaChamkhi]

Thanks for the feedback! I've addressed the review comments:

• Converted the example from a Flutter app to a Dart script
• Restored the README to its original content, only adding code-excerpt references