feat(gnoweb): add metadata to md content

[alexiscolin]

Fixes #3333

This PR introduces a way to customize HTML head metadata (title, description, and maybe soon, canonical) from realm markdown content. It adds a front-matter option that can be used at the very top of any realm markdown file. This front-matter is then read by our Markdown parser, Goldmark, to populate the Title and Description fields in gnoweb.

The PR provides default content where these fields were previously blank and maintains a fallback setup using h.Static.Domain + " - " + gnourl.Path for the Title.

Additionally, regardless of the provided Title, it will always be followed by - gnourl.Path ( - gno.land in our case).

For now, only Title and Description are supported. Any other variables in the front-matter are ignored.

Usage example:

---
Title: Mon Titre 
Description: Ma description 
---

[Gno2D2]

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[codecov]

Codecov Report

Attention: Patch coverage is 77.56098% with 46 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
gno.land/pkg/gnoweb/markdown/meta.go	80.25%	30 Missing and 1 partial ⚠️
gno.land/pkg/gnoweb/webclient_html.go	52.63%	6 Missing and 3 partials ⚠️
gno.land/pkg/gnoweb/handler.go	79.31%	2 Missing and 4 partials ⚠️

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[leohhhn]

Could you also add the front matter to some example realm to showcase the usage? 🙏

[aeddi]

I've never used goldmark or its extension system, but compared to the column PR, this one looks good to me.

[leohhhn]

lgtm

What will happen if i embed more data apart from title and description?

Also, since this can technically be changed after deployment with proper functionality in realms, will this have an impact on SEO or similar? Just considering different cases.

[alexiscolin]

lgtm

What will happen if i embed more data apart from title and description?

Also, since this can technically be changed after deployment with proper functionality in realms, will this have an impact on SEO or similar? Just considering different cases.

It is possible to add more variables in the frontmatter, but they will not be used by gnoweb. However, it would be appropriate to add others, such as canonical or even a style definition to customize the rendering within gnoweb.

While the frontmatter can be used to store any type of content, in this case, it is added specifically for gnoweb. For SEO and accessibility in this case. Therefore, aside from specific methods, continuing to use Markdown-native properties to manage what gnoweb renders from Markdown makes sense in my opinion.

[moul]

=> #3910