[doc] replace Mark-Down(MD) with RST(Read-The-Doc)

[leemgs]

In order to give users more convenience, let's introduce the Read-The-Doc service (e.g., readthedocs.io) with RST file format instead of the existing MD file format.

Case study

Powerline tmux plugin

• https://powerline.readthedocs.io/en/latest/installation.html
• https://github.com/powerline/powerline/blob/develop/docs/source/installation.rst

Linux kernel

• https://readthedocs.org/projects/kernel/
• https://kernel.readthedocs.io/en/latest/

[taos-ci]

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