Fix docify embed in md

[iulianbarbu]

When embedding Rust code samples in markdown files, this happens with Rust lang. If the READMEs are included as crate level docs for example, those code samples will represent down the line doc tests.

We want ideally for docify::embed to have the same functionality when used for markdown files as it has for regular .rs docs.

This PR adds support for multiple langs, specifically for the case of wrapping around Rust code samples with rust,ignore, and avoid having them interpreted as docs tests.

[iulianbarbu]

Hey @sam0x17 ! Can you take a look at this PR soonish and if all good merge/publish a new crate version for it? This fix is important for some workflows on polkadot-sdk where we want to ignore some rust docs tests, and I thought we might get it in and use it sometime next week, ideally by Wednesday when we'll have a release.

No problem at all if you can't, we have an ugly fix to use until this is released.

[sam0x17]

fix test failures and then I can merge

[iulianbarbu]

fix test failures and then I can merge

Thanks! Will do that soon.

[iulianbarbu]

@sam0x17, I think tests should pass now.