docs: add Maintainer Retainer Program (MRP) section

[danbaruka]

Description

Provide a clear and concise description of the changes in this PR.

• What problem does this PR solve?
  There was no dedicated documentation for the Maintainer Retainer Program (MRP) in the dev-ex repo. A reusable, draft section was needed for validation before adding it to CONTRIBUTING.md and other MRP repos.
• What is the purpose of this change?
  To add a short MRP overview (Core vs Community Maintainers, official source of truth) and a link preview card for the official OSC page, so contributors have one place to reference and so the text can be reused across repos after review.

Related Issue

Fixes #148

Type of Change

Check the type of change this PR introduces:

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactor (improves code structure or readability without changing functionality)
• Other (please describe):

Changes Made

What changes were made to the codebase?
Include details about files modified, functions/logic modified.

• Add MRP overview with link to official OSC page
  New doc website/docs/maintainer-retainer/maintainer-retainer.md: short overview of the MRP, definitions of Core and Community Maintainers, application callouts (Community Projects + Developers → Community Maintainers), and a clear link to the official OSC Maintainer Retainer page as the source of truth.

• Add link preview card with og:image and Intersect-style layout
  In the same doc, a link preview card for the official OSC page: blue title/URL above the card, card frame with the page’s og:image and gradient/grid styling (and dark-mode styles in website/src/css/custom.css).

• Category label: MRP; sidebar title from frontmatter
  New website/docs/maintainer-retainer/_category_.yml with sidebar label MRP and position. Doc uses frontmatter title: Maintainer Retainer Program so the sidebar shows that title and the page has a single clear heading.

Checklist

Ensure you have completed the following steps before submitting your PR:

• My code follows the project's coding style and guidelines.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation (if applicable).
• My changes generate no new warnings or errors.
• I have added tests that prove my fix is effective or that my feature works.
• I have checked for merge conflicts.
• I have rebased my branch onto the latest main or master branch.

[Ranchhand87]

Line 8: We need to add the abbreviation above in Title and/or label. Alternatively, Let's put 'Maintainer Retainer Program (MRP)' on this line.

[Ranchhand87]

Maybe we can tag some "core" maintainers here? I was thinking it could look more paragraphical? But that could be different RE: docs in various core projects.

To kick this off...
Cc: @lehins @ch1bo @sgillespie

[danbaruka]

Quick question: would you prefer this as a single concise paragraph, or two short paragraphs?

[Ranchhand87]

Quick question: would you prefer this as a single concise paragraph, or two short paragraphs?

Let's leave the initial format, add a one paragraph style, and a two paragraph style. We then provide optionality to participating projects to select the style that best matches their docs.

Thoughts?

[danbaruka]

This is a great idea. I’ll go ahead and add both options.

[danbaruka]

I’ve just added the two additional options.

[ch1bo]

Maybe we can tag some "core" maintainers here? I was thinking it could look more paragraphical? But that could be different RE: docs in various core projects.

To kick this off... Cc: @lehins @ch1bo @sgillespie

I'm sorry, but what exactly you need from me / why did you tag me?

[Ranchhand87]

@ch1bo

I'm sorry, but what exactly you need from me / why did you tag me?

Hi, This is currently a draft of a 'documentation update request' for the repositories/projects that are currently enrolled in the Maintainer Retainer Program. As each repo/project may have a slightly different documentation style, this is designed to serve as a reference with multiple options for how to adjust the current CONTRIBUTING.md file. Any input you may have would be helpful, A list of the enrolled projects can be found here: https://opensourcecommittee.docs.intersectmbo.org/about/paid-open-source-model-posm/maintainer-retainer/project-list