[R2] Add new tutorial for R2 event notification (cloudflare#17485)

* add new r2 event notification tutorial

* update frontmatter

* update the html code

* Update src/content/docs/r2/tutorials/summarize-pdf.mdx

Co-authored-by: Denise Peña <[email protected]>

* Apply suggestions from code review

Co-authored-by: Denise Peña <[email protected]>

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Co-authored-by: Denise Peña <[email protected]>