Doc test generator

[aumetra]

A quick little tool that visits all .mdx files, parses them using pulldown-cmark, and writes all codeblocks annotated with something like:

```rust template="core"

into a separate test file, that applies the core template.

There is nothing fancy about this, just some globbing, aggregating, and file creation.
Also supports Go blocks, but I'm not sure how to properly set up a working test environment for this, I'm just a poor little Rust dev.

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This should either be a Git hook or we should have a CI step that ensures no new files have been created (probably both).

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Name	Status	Preview	Comments	Updated (UTC)
cosmwasm-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	May 2, 2024 0:18am

[chipshort]

I think this gets the job done nicely. No need to be too fancy here.
Regarding CI: We should have a step that checks no new files are created and also run the tests.
CI should also have a cronjob that runs the tests with the latest versions of our crates, so we notice when it breaks with new releases.

[chipshort]

I'm not too familiar with Github Actions, but looks good to me.
Nice work and thanks for taking the initiative on this!