add exporters/markdown_multipage

[nodiscc]

• fixes new exporter: multi-page HTML #15
• add all tags to the sidebar
• add h1 headers to all tags pages
• append the number of items next to each tag in the sidebar
• Add stars count next to the name Feature request: Add stars and last modified date next to the name awesome-selfhosted/awesome-selfhosted#3019
• Add last update date next to the name Feature request: Add stars and last modified date next to the name awesome-selfhosted/awesome-selfhosted#3019
• If last update date is older than 6 months, render the date in orange
• If last update date is older than 12 months, render the date in red/dark orange
• remove right sidebar https://pradyunsg.me/furo/customisation/toc/
• add anti-features indicator
• filter out software with non-free licenses
• fix dead/relative links (https://myst-parser.readthedocs.io/en/latest/syntax/optional.html#syntax-substitutions ?)
• upload preview site (delete genindex.html? delete objects.inv?)
• add logo
• add favicon
• remove hotlinked images
• add robots.txt (Crawl-delay)
• render licenses list on main page
• render anti-features list on main page
• link licenses boxes to licenses list
• link anti-features boxes to anti-features list
• test accessiblity https://accessibilitytest.org/
• css: reduce margin of hr.docutils to margin: 1rem 0;
• add license to all page footers
• fix build warnings
• render clients links
• decrease .sidebar-brand-text font-size to 1.4rem (does not work in inline CSS)
• move inline CSS to custom CSS file? later, works as-is
• weird file path for AUTHORS file in sphinx html output
• add metadata to html pages https://myst-parser.readthedocs.io/en/latest/configuration.html#setting-html-metadata
• generate sitemap.xml? later, requires third-party sphinx extension
• add deployment instructions for the generated site
• fix AUTHORS file formatting
• fix bug in redirect admonition
• exporters/markdown_singlepage: generate non-free.md
• exporters/markdown_singlepage: allow not rendering tag/category header (description, related, redirect...)
• exporters/markdown_singlepage,multipage: only render licenses not matching exclude_licenses/matching include_licenses in the license list
• update exporters/markdown_singlepage documentation/examples in README.md
• importers/exporters: use flat files tags.yml,platforms.yml instead of subdirectories?