Serve Markdown when requested (#2181)

* Add support for rendering each page as md

* On md pages, change relative links to go to md version

* Netlify: respond with md if accept header equals text/markdown

* Render shortcodes

* Remove netlify accept header functionality

qdrant-landing/layouts/_default/single.markdown.md

@@ -0,0 +1,8 @@
+# {{ .Title }}
+
+{{- $content := .RenderShortcodes -}}
+{{- /* Rewrite internal absolute links: ](/path/to/page/) → ](/path/to/page/index.md) */}}
+{{- $content = replaceRE `\]\((/[^):]*/)` `](${1}index.md` $content -}}
+{{- /* Rewrite internal relative links: ](../page/) or (./page/) → same with index.md */}}
+{{- $content = replaceRE `\]\((\.\.?/[^):]*/)` `](${1}index.md` $content -}}
+{{ $content }}