minor symfony#17916 Update contributing docs (nicolas-grekas)

This PR was merged into the 2.3 branch.

Discussion
----------

Update contributing docs

| Q             | A
| ------------- | ---
| Branch        | 2.3
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

I propose adding the branch to the pull request template so that submitters have some hint about the branch selection workflow, and so that we can edit it for mergers.

I'd also like to add a link to the "running tests" doc, which made me rewrite the full contributing file, with links to the sections of the contributing documentation.

Commits
-------

852f999 Update contributing docs

Author: fabpot

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,5 +1,6 @@
 | Q             | A
 | ------------- | ---
+| Branch        | master for features and deprecations / lowest applicable and maintained version otherwise
 | Bug fix?      | yes/no
 | New feature?  | yes/no
 | BC breaks?    | yes/no

CONTRIBUTING.md

@@ -5,16 +5,20 @@ Symfony is an open source, community-driven project.
 
 If you'd like to contribute, please read the following documents:
 
-* [Contributing Code][1]: The document index related to contributions;
+* [Reporting a Bug][1]
+* [Submitting a Patch][2]
+* [Symfony Core Team][3]
+* [Security Issues][4]
+* [Running Symfony Tests][5]
+* [Our Backwards Compatibility Promise][6]
+* [Coding Standards][7]
+* [Conventions][8]
 
-* [Submitting a Patch][2]: Guidelines for submitting a pull request;
-
-* [Pull Request Template][3]: Template header to use in your pull request
-  description;
-
-* [Backwards Compatibility][4]: Backward compatibility rules.
-
-[1]: https://symfony.com/doc/current/contributing/code/index.html
-[2]: https://symfony.com/doc/current/contributing/code/patches.html#check-list
-[3]: https://symfony.com/doc/current/contributing/code/patches.html#make-a-pull-request
-[4]: https://symfony.com/doc/current/contributing/code/bc.html#working-on-symfony-code
+[1]: https://symfony.com/doc/current/contributing/code/bugs.html
+[2]: https://symfony.com/doc/current/contributing/code/patches.html
+[3]: https://symfony.com/doc/current/contributing/code/core_team.html
+[4]: https://symfony.com/doc/current/contributing/code/security.html
+[5]: https://symfony.com/doc/current/contributing/code/tests.html
+[6]: https://symfony.com/doc/current/contributing/code/bc.html
+[7]: https://symfony.com/doc/current/contributing/code/standards.html
+[8]: https://symfony.com/doc/current/contributing/code/conventions.html