Tutorial Chaining Tactics (#66)

* first partial draft of part1

* close section

* first draft of section 1

* add part 2.1

* second draft

* typos

* update web page

* fix gaetan's first comment

* fix gaetan's second comment

* fix gaetan's second comment 2

* fix coqdoc comment in the intro

* fix Lysxia comments

* fix Lyes Saadi's comments

* edit index