[Enhancement] update website

### Search before asking

- [x] I had searched in the [issues](https://github.com/apache/eventmesh/issues?q=is%3Aissue) and found no similar issues.

### Enhancement Request

he EventMesh website homepage has several navigation and layout issues that hurt discoverability and first impressions:

Broken documentation link (404): The "Explore A2A Documentation" entry points to /docs/design-document/a2a-protocol, but this page is missing from the v1.12.0 versioned docs snapshot, so users hit a 404.
No way to switch doc versions from the homepage: There is no version selector in the top navigation, so visitors cannot easily discover or jump to a specific release's documentation.
Navigation is hard to scan: Primary items are ordered inconsistently, related links (Documentation, Community, Blog) are not grouped logically, and there is no quick jump from the navbar into key homepage sections such as the Agent Mesh / A2A and Ecosystem areas.
Overlapping/unreadable dropdowns: The hero heading sits too close to the fixed navbar, and the homepage dropdown menus use a near-transparent background, so when a menu expands it visually bleeds into the hero text and becomes hard to read.
Stale hero content: The hero release badge ("v1.12.0 Released — Now with A2A Protocol Support") is redundant once release notes/blog cover it and clutters the first view.
These issues affect both the English and Chinese (/zh/) homepages, as well as the navbar shown on internal documentation pages.

### Describe the solution you'd like

Improve the homepage navigation, layout, and documentation entry points consistently across EN/ZH and the doc pages:

Fix the A2A doc 404 — Add the A2A protocol document to the v1.12.0 versioned docs (EN + ZH) and register it in the versioned sidebar so /docs/design-document/a2a-protocol resolves correctly.
Add a dynamic version dropdown — Merge the docs version list into a single "Documentation" navbar dropdown, driven dynamically by the Docusaurus version data (so new releases appear automatically), applied consistently to the homepage (EN/ZH) and internal doc pages.
Add an "Agent Mesh" dropdown — Introduce an "Agent Mesh" navbar item whose toggle links to the A2A section, with an "Event Bus" sub-item that jumps to the Ecosystem section.
Reorder navigation for clarity — Adjust the item order so related links are grouped logically (e.g. Documentation ↔ Community and Blog ↔ Documentation swapped into a more intuitive layout).
Fix hero spacing and dropdown readability — Increase the hero's top padding so content is not hidden behind the fixed navbar, and give dropdown menus a solid, opaque background so they no longer overlap or bleed through the hero text.
Remove the stale hero badge — Drop the "v1.12.0 Released…" hero badge from both the EN and ZH homepages.
Acceptance criteria

/docs/design-document/a2a-protocol returns 200 in both EN and ZH.
The Documentation dropdown lists all versions and updates automatically on new releases.
Navbar (homepage EN/ZH + doc pages) shows the new order, the Agent Mesh → Event Bus dropdown, and no longer displays the hero badge.
Dropdown menus are fully opaque and readable, with no overlap between the hero heading and the navbar.

### Are you willing to submit PR?

- [x] Yes I am willing to submit a PR!

### Code of Conduct

- [ ] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct) *

Start at the Docusaurus navbar configuration, the English and Chinese homepage components, and the v1.12.0 versioned docs and sidebar referenced in the issue. Verify the A2A documentation path in both languages, then check the homepage and internal doc-page navigation against the acceptance criteria for version switching, ordering, dropdown readability, spacing, and badge removal.

Written by the indexing model from the issue text.