[doc][reorg] Move survey into xephonhq/notes-on-tsdb

[at15]

Currently the doc folder contains most up to date (though still outdated) survey on some tsdbs.
It's better to keep them in a dedicated survey repo. As I revived https://github.com/xephonhq/notes-on-tsdb just now, non libtsdb-go specific doc will be moved to that repo.

Might merge #22 first because I want to the book repo to contains those live code samples.
Which might explains some algorithms better.

[at15]

Reopen because It's not fully merged, I just want to work on the master branch of notes-on-tsdb and merged xephonhq/notes-on-tsdb#3