Add a global search bar

### Epic

_No response_

### Description

Following the implementation of https://github.com/FlowFuse/website/issues/2630, all search bars are currently namespaced and only show results from their current scope (blog, docs, handbook).

Given that the search bar is the top interacted with element from our website, we should add a global search bar that will bring relevant results while also making use of algolia's search capabilities in full (eg filtering and grouping results by category/scope)

examples
https://codesandbox.io/p/sandbox/github/algolia/autocomplete/tree/next/examples/instantsearch
https://codesandbox.io/embed/github/algolia/instantsearch/tree/master/examples/js/getting-started
https://www.algolia.com/doc/guides/building-search-ui/widgets/showcase/js/

Available scopes that we can search in:
'ama': 'Ask Me Anything',
'blog': 'Blog',
'changelog': 'Changelog',
'customer-stories': 'Customer Stories',
'docs': 'Docs',
'ebooks': 'E-Books',
'handbook': 'Handbook',
'node-red': 'Node-RED',
'webinars': 'Webinars'

### Acceptance Criteria

_No response_

### Have you provided an initial effort estimate for this issue?

I have provided an initial effort estimate