Documentation updates for version 1.28.0

[jamie-lemon]

No description provided.

[Copilot]

Pull request overview

This PR updates the Sphinx documentation to reflect v1.28.0 additions around Markdown support and CSS styling, and to centralize / cross-link conversion and styling guidance across the docs.

Changes:

• Adds documentation for Markdown-related workflows (Markdown → PDF, CSS styling via apply_css, archive usage).
• Adds a new “CSS Support” reference section and links to it from Story / Page documentation.
• Updates feature/support matrices and icons to include Markdown (MD) as a supported input type.

Reviewed changes

Copilot reviewed 11 out of 12 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
docs/the-basics.rst	Replaces a conversion recommendation with a link to the central converting guide.
docs/supported-files-table.rst	Adds an MD icon entry and corresponding CSS for the supported file types table.
docs/recipes-stories.rst	Replaces inline CSS limitations with a link to the CSS support reference.
docs/page.rst	Links insert_htmlbox CSS parameter docs to the CSS support reference.
docs/images/icons/icon-md.svg	Adds the Markdown icon asset used by matrices/tables.
docs/how-to-open-a-file.rst	Adds empty-document open example and an “open() options” section.
docs/document.rst	Documents archive in the constructor signature and introduces Document.apply_css().
docs/converting-files.rst	Expands conversion docs with a Markdown → PDF section (paper size, CSS, fonts) and reorders SVG section.
docs/archive-class.rst	Updates Archive description to include usage when opening files.
docs/app3.rst	Adds a new “CSS Support” appendix section listing supported/unsupported CSS features.
docs/about.rst	Updates product comparison to include MD as an input document type.
docs/about-feature-matrix.rst	Adds the MD icon embedding/CSS and shows MD in the feature matrix.

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