Changes

Updated StructuredText to match StructuredText component in @datocms/svelte (there currently is no official DatoCMS Astro package available), including:
- Extensible setup with support to pass custom components for rendering nodes.
- Typings, resolving linting issues current StructuredText.
- Documentation with usage and examples.
Moved custom components for TextBlock to that component.
Added custom component for rendering Heading with a minimum level of 2, avoiding multiple H1's on the page.
Added missing component for ThematicBreak node.

Associated issue

N/A

[x] I have performed a self-review of my own code
[x] I have made sure that my PR is easy to review (not too big, includes comments)
~~I have added/updated tests to prove that my feature works (if not possible please explain why)~~
[x] I have made changes to the README and if the change affects the project setup (npm commands changed, new service added, environmental variable added)
~~I have added a decision log entry if the change affects the architecture or changes a significant technology~~
[x] I have notified a reviewer
~~I respect off days and set a good example by not working during holidays for my colleagues~~

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