Adds strapi guide (platformsh#3483)

* Upsun Express Guide v0.1

* FHK: refacto Express guide

* adding title shortcode code-link + improve Express guide

* Express guide v1.0

* rework on Express guide

* adding .gitignore file + small improvements

* changing 'Git Flow' to 'Git workflow'

* adding project:init screenshot + explain what it does

* changing folder to my-express-app and add it in the folder schema

* missing vendorisation

* adding link to tutorial on how to create a project from the console

* Adding a note telling readers that they can add services during project:init and link to add a database

* remove git add . references to list exact files to commit

* Add a database finished + adding 'Terminal' tab for all of the shell command snippets

* missing terminal tab info

* adding Add a database steps number

* adding local development guide

* remove useless envVar from .environment

* Anouck's review - solve issues

* Anouck's review - solve issues

* remove extra lines to perform a redeploy :P

* change .environment for having RELATIONSHIPS_JSON: empty by default during project:init with no service

* solve Paul's review

* adding first steps to deploy Strapi on Upsun, needs to finish the modify your code part

* align index.md with Express one

* finishing the Strapi guide

* adding modification in a feat-a branch

* remove useless characters

* Apply suggestions from code review

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Co-authored-by: AnouckColson <[email protected]>