feat(mdx): Add estimated reading time

[unrealhoang]

to show the estimate read time of each post

[thien-do]

Could use something on top of https://github.com/syntax-tree/hast-util-reading-time

[thien-do]

Hm this turns out is not straightforward. The available tools are:

• https://github.com/syntax-tree/hast-util-reading-time too low level
• https://github.com/mattjennings/remark-reading-time use "reading-time" instead of the official low level tool
• https://github.com/rehypejs/rehype-infer-reading-time-meta doesn't expose the info for us to use

I intended to mark this as good first issue but look like we will need to wait a bit more