Tutorial about Intro patterns (#73)

* sec1

* write most of sec 3 and 4

* some work on section 5

* first draft

* first draft 2

* typo

* edit index

* Fix Pierre Courtieu's comments

* separate example of sec5 in two part => rewriting sec3 & views sec5

* Apply suggestions from code review

Co-authored-by: Quentin VERMANDE <[email protected]>

* add ex Exists

* Apply suggestions from code review

Co-authored-by: Pierre Rousselin <[email protected]>

* Apply suggestions from code review

* add Pierre Rousselin's example

* add parentheses

* add Lyes Saadi's comment

* Add other tactics with as

also correct some whitespace issues and add a small remark after the
first "destruct as".

* Update src/Tutorial_intro_patterns.v

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Co-authored-by: Quentin VERMANDE <[email protected]>
Co-authored-by: Pierre Rousselin <[email protected]>
Co-authored-by: Pierre Rousselin <[email protected]>