docs: add A2A & generative UI hackathon banner

[benclarkeio]

Summary

Add an announcement banner promoting the A2A and generative UI hackathon in London on Saturday 13th June, linking to the Luma signup page.

The existing DeepLearning.AI course banner is commented out for now so a single, focused announcement is shown for the next 10 days. After the hackathon, removing the hackathon <p> and uncommenting the original course banner restores the previous state exactly — see "Future state" below.

Changes

Only two files are touched:

• .mkdocs/overrides/main.html — add a new <p class="md-banner__hackathon"> for the hackathon announcement; comment out the previous course-banner content.
• docs/stylesheets/custom.css — replace the previously dead .announce .md-button rule with a :has(.md-banner__hackathon)-scoped layout that right-aligns the announcement button (with smaller padding) and vertically centers the dismiss icon.

Before / After

Before (current production banner)

After (this PR)

Future state (after the hackathon)

The CSS is scoped via :has(.md-banner__hackathon), so when the hackathon paragraph is later removed and the course banner is uncommented, all custom styling becomes inert and the course banner renders exactly as it did before this PR (default Material styling, big inline "Enroll for free" button, ✕ floated top-right). Verified locally by reverting the template to its pre-PR contents — the page rendered identically to the current production site.

Dismiss behaviour

Material's announce.dismiss keys dismissal off a hash of the announce block's HTML, so users who previously dismissed the course banner will still see the new hackathon banner (different content → different hash → different dismissal state). The same applies when the course banner is reinstated after the hackathon.

Test plan

• Build the docs locally with ./scripts/build_docs.sh — no warnings introduced.
• Serve with mkdocs serve and verify the hackathon banner renders with the button right-aligned, vertically centered, and the ✕ vertically centered with a small gap (see "After" screenshot).
• Simulate the post-hackathon state by reverting .mkdocs/overrides/main.html to its pre-PR contents; confirm the page renders identically to the current production site (default Material banner styling).
• Confirm custom.css changes don't affect any other .md-button on the site — the new selectors are all scoped under .md-banner__inner:has(.md-banner__hackathon) / .md-banner__hackathon.

[gemini-code-assist]

Code Review

This pull request replaces the DeepLearning.AI course announcement banner with a new banner promoting the "A2A and generative UI hackathon" in London, along with corresponding CSS updates for layout and responsiveness. The reviewer suggested several improvements: adding flex-wrap: wrap and flex-shrink: 0 to the CSS to prevent layout overflow on smaller screens, adding target="_blank" and rel="noopener" to the external Luma link to open it in a new tab, and using Jinja2 comments ({# ... #}) instead of HTML comments to completely omit the commented-out block from the rendered output.

Important

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[benclarkeio]

Note on the failing Check linked issues check: there's no existing issue tracking this hackathon announcement, so I haven't linked one. The workflow allows bypassing via the no-issue label (per skip-linked-issues-check-label: no-issue) — could a maintainer apply that if appropriate? Happy to open a tracking issue first if you'd prefer.

(The spelling and Lint Code Base failures are addressed in the follow-up commit 897443a.)

[muscariello]

LGTM

[msampathkumar]

LGTM.