Add a markdown-doctest CI for docs directory

[harendra-kumar]

[adithyaov]

The markdown-doctest executable that we have is

1. Hacky
2. Depends on ghcid library

Depending on the ghcid library is a roundabout way of doing things. Instead, we should use streamly-process to pipe the input to a ghci session.

Implementing this will take time. And since this is not an urgent issue, I'm pushing it to the next milestone.