[feat] Collapsible table of content headings

[kiranparajuli589]

if a blog wraps smaller headings inside a big headings like:

...
# blog title
...
## heading 1
...
### sub heading 1
...
## heading 2

then the table of content should be made collapsible w.r.t to the headings like:

Table of content
---------------------
➡️ heading 1 ⬇️ 
    ➡️ sub heading 1
➡️ heading 2