I use Org but markdown-mode can hide subsections too: markdown-mode supports outline-minor-mode as well as org-mode-style visibility cycling for atx- or hash

I use Org but markdown-mode can hide subsections too: markdown-mode supports outline-minor-mode as well as org-mode-style visibility cycling for atx- or hash-style headings. There are two types of visibility cycling: Pressing S-TAB cycles globally between the table of contents view (headings only), outline view (top-level headings only), and the full document view. Pressing TAB while the point is at a heading will cycle through levels of visibility for the subtree: completely folded, visible children, and fully visible. (Quote from the link shared by parent.)