docs: add research seminar deck

[Maokami]

Summary

• add a separate research-seminar deck under site/AbsinterpSlides/Seminar.lean
• publish it under /seminar/ while keeping the short overview deck at /slides/
• wire the new deck into the docs-site build and Home page

Validation

• cd site && lake build
• cd site && lake exe seminar-slides

Notes

• this implements issue Add a research-seminar slide deck for absinterp #121
• the short overview deck remains intact
• the seminar deck is aimed at a 20–30 minute research talk for Lean users and PL / static-analysis researchers

[gemini-code-assist]

Code Review

This pull request introduces a new research-seminar slide deck for the absinterp project. The changes include the creation of the seminar content in site/AbsinterpSlides/Seminar.lean, a new entry point in site/SeminarMain.lean, and updates to the build configuration in site/lakefile.toml to support the seminar-slides executable. Additionally, the documentation in site/README.md and site/AbsinterpDocs/Home.lean has been updated to include information and links for the new deck. I have no feedback to provide as there were no review comments.