First draft of "Deploying to Heroku Cloud" cookbook article
Fixed som RST syntax errors
Added the new cookbook to the article map
Fixed all problems reported by @WouterJ
Fixed the target of one link
Applied all the fixes suggested by @xabbuh
feature #4045 Added a new "Deploying to Heroku Cloud" cookbook article (javiereguiluz)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #4045).

Discussion
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Added a new "Deploying to Heroku Cloud" cookbook article

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | master
| Fixed tickets | -

Commits
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0a1ba36 Applied all the fixes suggested by @xabbuh
87c0d08 Fixed the target of one link
2fdfd6d Fixed all problems reported by @WouterJ
31003b2 Added the new cookbook to the article map
d76bf8e Fixed som RST syntax errors
0371402 First draft of "Deploying to Heroku Cloud" cookbook article