Support `<br>` in docs-builder

[bmorelli25]

Summary

We need to support the ability to add line breaks. The <br> HTML tag is a common method of supporting this feature in md authoring tools. We also use <br> in the asciidoc system, so it would migrate over nicely.

[reakaleek]

I'm not sure if we can add a custom implementation for this.

I found that to be able to support this natively we need to remove the .DisableHtml() method at https://github.com/elastic/docs-builder/blob/main/src/Elastic.Markdown/Myst/MarkdownParser.cs#L55

@Mpdreamz WDYT?

What was the reason for the decision to disable html?

P.S.
I understood now that we can extend e.g. the HtmlBlockParser

[Mpdreamz]

We want to be prescriptive with styling and layout so therefor we'll never allow html.

To support the br tag in isolated we need to create a new inline markdown parser that injects it to the markdown AST as its own leaf element.

Then we can render it like all the other markdown constructs and write the br html element.

[reakaleek]

Thank you for the hint @Mpdreamz.

Will be fixed by #342

[reakaleek]

It's merged now and will be in the next release.

One note, it only supports exactly <br>.

<br/> will not work and will be rendered as text.