Add detailed Laravel Excel Boost skill references

[eramitgupta]

Adds package-specific Boost skill references for Laravel Excel.

This change keeps resources/boost/skills/laravel-excel/SKILL.md focused on the core workflow and adds references/package.md for deeper package guidance. The reference covers generator commands, export/import concerns, queueing, config, macros, testing patterns, and common failure modes so agents can load detailed context only when needed.

1. Why should it be added? What are the benefits of this change?

It improves the Laravel Excel Boost skill by separating the quick-start guidance from deeper package reference material. That reduces context bloat in SKILL.md while giving agents better package-specific detail when working on imports, exports, queueing, and testing.

2. Does it contain multiple, unrelated changes? Please separate the PRs out.

No. This PR only adds Laravel Excel skill documentation and its related reference file.

3. Does it include tests, if possible?

No runtime code changed, so no package tests were added. The added files were checked for required Boost skill structure and reference linking.

4. Any drawbacks? Possible breaking changes?

No breaking changes expected. This only adds documentation files under resources/boost/skills/laravel-excel/.

5. Mark the following tasks as done:

• Checked the codebase to ensure that your feature doesn't already exist.
• Take note of the contributing guidelines.
• Checked the pull requests to ensure that another person hasn't already submitted a fix.
• Added tests to ensure against regression.