update best practices index

This way, the table of contents on the index page has only one level
like every other index in the documentation.
Correct link to scopes page
changes from xabbuh

change the link according to xabbuh comment
minor #4437 Correct link to scopes page (mayeco)

This PR was submitted for the 2.5 branch but it was merged into the 2.3 branch instead (closes #4437).

Discussion
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Correct link to scopes page

The current link to scopes page dont work, this is the correct link to the page

Commits
-------

7a37cb8 changes from xabbuh
434fed7 Correct link to scopes page
minor #4352 [Best Practices] update best practices index (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
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[Best Practices] update best practices index

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

This way, the table of contents on the index page has only one level
like every other index in the documentation.

Commits
-------

facea46 update best practices index
removed references to documentation from external sources
Removed more references to third party docs
removed more references to documentation from external sources
minor #4106 removed references to documentation from external sources (fabpot, WouterJ)

This PR was merged into the 2.3 branch.

Discussion
----------

removed references to documentation from external sources

| Q             | A
| ------------- | ---
| Doc fix?      | n/a
| New docs?     | n/a
| Applies to    | all
| Fixed tickets | n/a

Referencing external doc sources was a mistake as the versioning can be (and is) different from the versioning of the Symfony docs.

**WARNING** I will merge this one myself to coordinate the merge with the release of the new frontend on symfony.com.

Commits
-------

2cb8d27 removed more references to documentation from external sources
c39ea1d Removed more references to third party docs
0e2c491 removed references to documentation from external sources
Merge branch '2.3' into 2.5

* 2.3:
  removed more references to documentation from external sources
  Removed more references to third party docs
  removed references to documentation from external sources
  changes from xabbuh
  Correct link to scopes page
  update best practices index