Fixes or removes a good handful of broken links found from testing
#1335, as well as some redirects / domain changes.

- `angular.io` moved to `angular.dev`
- Tried to find the closest match to any angular docs/guides that had
been moved
- `gatsbyjs.org` moved to `gatsbyjs.com`
- One link to `intro-to-web-accessability` (misspelt) in
art-of-a11y-preface
- A download link to some presentation slides (broken since the content
refactor) in angular-templates-start-to-source
- A number of deleted blog posts on external sites

Fixes #1233, but differently™

This PR changes the unified processor to return a "component tree"
rather than a plain HTML string. This tree contains portions of raw HTML
as well as props that would normally be passed to interactive components
(tabs, file lists, code embeds, link previews, etc).

The component tree is then passed to an Astro component, which is used
recursively to render the raw HTML and components.

This results in any imports (styles & scripts) being conditional on the
component appearing in the post. It also means that scoped CSS and
client hydration are supported.

In the future, this component tree could be cached with the post
content, and any SSR-dependent behavior could happen in the Astro
components.

## Changes to note:

- Components can now _only_ be nested at the root level of other
components, or within specific elements that are transformed into
components (`<details>`)

This means that iframes can no longer be placed within a list, as was
done in `example/index.md`. When this happens, the transformer will log
& throw an error.

- EPub generation is moved from a separate build script to a
`pages/[slug].epub.ts` Astro route
(this means it isn't parallelized, but takes advantage of caching & will
massively improve the build time with #1335)

- Conditional `tabs-small` behavior (keeping tabs at a fixed height) is
removed

### TODO:

- [x] Migrate tabs to preact
- [x] Migrate in-content-ad to preact
- [x] Migrate link-preview to preact
- [x] Migrate filetree to preact
- [x] Migrate IFramePlaceholder to preact
- [x] Migrate hints to preact
- [x] Validate epub changes