Markdown style link for mdbook

[ordovicia]

Raw URL strings (e.g. https://example.com) are not recognized as links in published mdbook.
So this PR converts them into Markdown style (e.g. [link](https://example.com)).

[steveklabnik]

You could also use <http://whatever.com>, which is a bit shorter. I guess you've already done the work...

[ordovicia]

Ah I did not know that. Thank you for telling me.
Actually I did this work with a short script, so it was not a hard job.

Also, in some RFCs links were written in [link](https://example.com) style originally, and I think it's better to unify styles.

[Centril]

Thanks! Merging for now - we can always change to the other format later.