Add GraphQL field `readme_html` to CiCatalogResource

What does this MR do and why?

Added the readme_html field to ciCatalogResource in GraphQL.

This field returns the content of the catalog resource's README file converted from markdown to HTML.

Resolves #408721 (closed).

How to set up and validate locally

1. In the Rails console, run Feature.enable(:ci_private_catalog_beta) and Feature.enable(:ci_namespace_catalog_experimental).
2. Create a Catalog Resource with an existing or new Project:

::Ci::Catalog::Resource.create(project: Project.find(<project_id>))

3. Ensure your Project contains a README.md file with some GFLM content. Sample content:

# Test Header

**Test Bold Text**

4. Go to http://gdk.test:3000/-/graphql-explorer and run the following query (adjust the projectPath as necessary):

query getCiCatalogResources {
  ciCatalogResources(projectPath: "group-a/project-catalog-resource-2") {
    nodes {
      id
      name
      readmeHtml
    }
  }
}

5. Observe that the output shows the README.md content converted to HTML.

Screenshot

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #408721 (closed)