update contribution guide for 2.7/3.0
Fixed some code indentation
Fix indentation of YAML example
Linked the PDO/DBAL Session article from the Doctrine category
minor #4649 Linked the PDO/DBAL Session article from the Doctrine category (javiereguiluz)

This PR was squashed before being merged into the 2.3 branch (closes #4649).

Discussion
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Linked the PDO/DBAL Session article from the Doctrine category

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | #4416

Commits
-------

cb4784a Linked the PDO/DBAL Session article from the Doctrine category
replace Symfony2 with Symfony
minor #4652 replace Symfony2 with Symfony (xabbuh)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #4652).

Discussion
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replace Symfony2 with Symfony

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

Commits
-------

224c283 replace Symfony2 with Symfony
minor #4659 Fixed some code indentation (javiereguiluz)

This PR was merged into the 2.3 branch.

Discussion
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Fixed some code indentation

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

Commits
-------

0b7cb0e Fixed some code indentation
Update the_controller.rst

Corrected error in the sample code.
bug #4646 Update the_controller.rst (teggen)

This PR was submitted for the 2.6 branch but it was merged into the 2.3 branch instead (closes #4646).

Discussion
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Update the_controller.rst

Corrected error in the sample code.

Commits
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40e63b9 Update the_controller.rst
minor #4660 Fix indentation of YAML example (xelaris)

This PR was merged into the 2.3 branch.

Discussion
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Fix indentation of YAML example

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

Commits
-------

52a05c3 Fix indentation of YAML example
Update controllers.rst
minor #4647 Update controllers.rst (keefekwan)

This PR was submitted for the 2.6 branch but it was merged into the 2.3 branch instead (closes #4647).

Discussion
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Update controllers.rst

Added in Route/ParamConverter use statements and @ParamConverter annotation

Commits
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1a0534d Update controllers.rst
Spelling mistake tens to tons
minor #4664 Spelling mistake tens to tons (albabar)

This PR was submitted for the 2.6 branch but it was merged into the 2.3 branch instead (closes #4664).

Discussion
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Spelling mistake tens to tons

I'm not sure, but I think it should be tons, not tens.

Commits
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299cc2e Spelling mistake tens to tons
bug #4513 [Contributing] update contribution guide for 2.7/3.0 (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
----------

[Contributing] update contribution guide for 2.7/3.0

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | #4508

Commits
-------

87582a4 update contribution guide for 2.7/3.0
consistency, replace "e-mail" with "email"
consisteny, use "front-end" instead of "frontend" or "front end"
consistency, replace "save-handler" with "save handler"
unify "Symfony Standard Edition" usages