Enhance LLM readable markdown rendering

[reakaleek]

Closes #1504
Closes https://github.com/elastic/docs-eng-team/issues/161

Changes

This adds a new LlmMarkdownRenderer that extends Markdig's TextRendererBase.
Hence, we can take an existing MarkdownDocument and render it with our new custom renderer.

Notes

The markdown is accessible in local serve, on previews as well as assembler builds.
You just need to append .md to the current URL.

Preview

https://docs-v3-preview.elastic.dev/elastic/docs-builder/pull/1569/syntax/quick-ref.md

[Mpdreamz]

LGTM! One nit around object pooling.

Did not go into output too much, we can update that as we continue to work closer with other teams.

[Mpdreamz]

We have an object pool for stringwriters and stringbuilders and htmlrenderes which can drastically help reduce allocation e.g:

See:

docs-builder/src/Elastic.Markdown/Helpers/DocumentationObjectPoolProvider.cs

Lines 43 to 45 in bdfb9ca

	var stringBuilder = StringBuilderPool.Get();
	using var stringWriter = StringWriterPool.Get();
	stringWriter.SetStringBuilder(stringBuilder);

For a usage example see:

docs-builder/src/Elastic.Markdown/Myst/Directives/DirectiveViewModel.cs

Line 16 in cb72d5e

var subscription = DocumentationObjectPoolProvider.HtmlRendererPool.Get();

[reakaleek]

I tried to create a LlmMarkdownRenderer object pool based on the existing HtmlRenderSubscription.

Unfortunately, I don't think it will work because I have the BuildContext as property on the LlmMarkdownRenderer.

But I guess I can still use it for StringWriter or StringBuilder.

WIP.

[reakaleek]

b6d8419

[reakaleek]

Ok. I just noticed, what I did is not enough. I need to return the object.

[reakaleek]

OK. I think I got it right now.

f975abd

[reakaleek]

OK OK. Now I think it's nice. 9e5d96d

[Mpdreamz]

On ultra nitpick but LGTM!