docs: update symfony documentation config for dev 5.x (#2426)

## Description

Update documentation config for 5.x

## What type of PR is this? (check all applicable)
- [ ] Bug Fix
- [ ] Feature
- [ ] Refactor
- [ ] Deprecation
- [ ] Breaking Change
- [ ] Documentation Update
- [ ] CI

## Checklist
- [ ] I have made corresponding changes to the documentation (`docs/`)
- [ ] I have made corresponding changes to the changelog
(`CHANGELOG.md`)

.symfony.bundle.yaml

@@ -1,6 +1,6 @@
-branches: ["2.x", "3.x", "master"]
-maintained_branches: ["3.x", "master"]
+branches: ["2.x", "3.x", "master", "5.x"]
+maintained_branches: ["master", "5.x"]
 current_branch: "master"
-dev_branch: "master"
-dev_branch_alias: "4.x"
-doc_dir: { "3.x": "Resources/doc/", "master": "docs/" }
+master_alias: "4.x"
+dev_branch: "5.x"
+doc_dir: { "3.x": "Resources/doc/", "master": "docs/", "5.x": "docs/" }