Add overview lectures to website (closes #18)

[mmcky]

Summary

• Adds an Overview Lectures section to website/index.html, placed between About and Table of Contents — reflects John's note in Add overview lectures to the website #18 that the two lectures are an overview of the whole project, not paired with a specific volume.
• Mirrors lecture1.pdf and lecture2.pdf into pdf/ as build artifacts (initial copy from book-dp2/presentation/).
• Follow-up issue to be filed on book-dp2 to wire these into the existing sync.yml so future updates propagate automatically, and to make a final call on source-of-truth.

Resolves the interim placement question raised in #18 without waiting for the broader MyST migration / website re-jig.

Test plan

• Render website/index.html locally and confirm the Overview Lectures section appears below About
• Both lecture PDF buttons resolve once the branch is merged to main (raw GitHub URLs become live)
• No regression to existing Vol I / Vol II tabs or Download buttons

Closes #18

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[Copilot]

Pull request overview

Adds an “Overview Lectures” section to the landing page so the two project-wide lecture PDFs are discoverable in a dedicated spot (between About and Table of Contents), consistent with the intent described in issue #18.

Changes:

• Adds a new “Overview Lectures” section on website/index.html.
• Adds two buttons linking to lecture1.pdf and lecture2.pdf under pdf/ via the repo’s existing GitHub raw URL pattern.

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[mmcky]

@jstac this PR adds a section below the Table of Contents and above Downloads.

[mmcky]

@jstac I have opened https://github.com/QuantEcon/book-dp2/issues/153 to fix the auto-sync from book-dp2.

Alternatively, would you like to move the presentations here?

I think not -- sync -- because all other sources of truth are based in book-dp2 and book-dp1

[jstac]

Thanks @mmcky , no need for autosync on those lectures.

I doubt they will change.