exporters/markdown_multipage: render subpages for each platform

[nodiscc]

• refactor render_tag_page as render_item_page and allow rendering either a tag or platform page
• link to each platform page from platform spans in the software item template
• ref. https://github.com/awesome-selfhosted/awesome-selfhosted-data/issues/71
• mark platform pages as orphans to prevent 'WARNING: document isn't included in any toctree' warnings