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]

🛠 PR Checks Summary

🔴 Pending initial approval by a review team member, or review from tech-staff

Manual Checks (for Reviewers):

• IGNORE the bot requirements for this PR (force green CI check)

Read More

🤖 This bot helps streamline PR reviews by verifying automated checks and providing guidance for contributors and reviewers.

✅ Automated Checks (for Contributors):

🟢 Maintainers must be able to edit this pull request (more info)
🔴 Pending initial approval by a review team member, or review from tech-staff

☑️ Contributor Actions:

1. Fix any issues flagged by automated checks.
2. Follow the Contributor Checklist to ensure your PR is ready for review.
   • Add new tests, or document why they are unnecessary.
   • Provide clear examples/screenshots, if necessary.
   • Update documentation, if required.
   • Ensure no breaking changes, or include BREAKING CHANGE notes.
   • Link related issues/PRs, where applicable.

☑️ Reviewer Actions:

1. Complete manual checks for the PR, including the guidelines and additional checks if applicable.

📚 Resources:

• Report a bug with the bot.
• View the bot’s configuration file.

Debug

Automated Checks

Maintainers must be able to edit this pull request (more info)

If

🟢 Condition met
└── 🟢 And
    ├── 🟢 The base branch matches this pattern: ^master$
    └── 🟢 The pull request was created from a fork (head branch repo: alexiscolin/gno)

Then

🟢 Requirement satisfied
└── 🟢 Maintainer can modify this pull request

Pending initial approval by a review team member, or review from tech-staff

If

🟢 Condition met
└── 🟢 And
    ├── 🟢 The base branch matches this pattern: ^master$
    └── 🟢 Not (🔴 Pull request author is a member of the team: tech-staff)

Then

🔴 Requirement not satisfied
└── 🔴 If
    ├── 🔴 Condition
    │   └── 🔴 Or
    │       ├── 🔴 At least 1 user(s) of the organization reviewed the pull request (with state "APPROVED")
    │       ├── 🔴 At least 1 user(s) of the team tech-staff reviewed pull request
    │       └── 🔴 This pull request is a draft
    └── 🔴 Else
        └── 🔴 And
            ├── 🟢 This label is applied to pull request: review/triage-pending
            └── 🔴 On no pull request

Manual Checks

**IGNORE** the bot requirements for this PR (force green CI check)

If

🟢 Condition met
└── 🟢 On every pull request

Can be checked by

• Any user with comment edit permission

[codecov]

Codecov Report

Attention: Patch coverage is 76.09756% with 49 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
gno.land/pkg/gnoweb/markdown/meta.go	78.34%	32 Missing and 2 partials ⚠️
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 ⚠️

📢 Thoughts on this report? Let us know!

[leohhhn]

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