Grav 2.1 adds native Markdown output and llms.txt support

Grav 2.1 adds core Markdown output: appending .md to a page URL, or requesting Accept: text/markdown, returns the rendered main content with YAML frontmatter, navigation links and resolved URLs.

Sitemap 5.3.0 adds /llms.txt, which indexes sitemap pages through their Markdown URLs, plus an optional cached /llms-full.txt.

The release also adds Markdown Twig customization and @grav template inheritance, while fixing Markdown Extra raw HTML handling, image-cache clearing during updates, invalid Flex markers in feeds, and several Admin, compression, permissions and Twig issues.