doc: Reorganize the developers documentation

I felt like some of the documents were not useful on a day-to-day basis.
Having an "Architecture" section is great for people who want to
understand the technical implementation of Bileto.

I also took the opportunity to homogeneize some titles.

docs/developers/README.md

@@ -3,18 +3,13 @@
 - [Setup the development environment](/docs/developers/setup.md)
 - [Update the development environment](/docs/developers/update.md)
 - [Executing tests and linters](/docs/developers/tests.md)
-- [Managing the dependencies](/docs/developers/dependencies.md)
-- [Understanding GreenMail](/docs/developers/greenmail.md)
-- [Using the translations](/docs/developers/translations.md)
-- [Working with the roles & permissions](/docs/developers/roles.md)
+- [Translating Bileto](/docs/developers/translations.md)
 
 Dedicated to the backend:
 
 - [Declaring a new Entity](/docs/developers/entity.md)
-- [Working with the search engine](/docs/developers/search-engine.md)
 - [Encrypting data](/docs/developers/encryptor.md)
-- [The email collector](/docs/developers/email-collector.md)
-- [Sort the entities](/docs/developers/sorters.md)
+- [Sorting the entities](/docs/developers/sorters.md)
 
 Dedicated to the frontend:
 
@@ -26,4 +21,12 @@ Dedicated to the frontend:
 
 Dedicated to the maintainers:
 
+- [Managing the dependencies](/docs/developers/dependencies.md)
 - [How to release a version](/docs/developers/release.md)
+
+Architecture:
+
+- [Email collector](/docs/developers/email-collector.md)
+- [GreenMail](/docs/developers/greenmail.md)
+- [Roles & permissions](/docs/developers/roles.md)
+- [Search engine](/docs/developers/search-engine.md)

docs/developers/email-collector.md

@@ -1,4 +1,4 @@
-# The email collector
+# Email collector
 
 Bileto allows you to configure mailboxes to receive emails.
 These emails are retrieved with <abbr>IMAP</abbr> and can be transformed in tickets.

docs/developers/greenmail.md

@@ -1,4 +1,4 @@
-# Understanding GreenMail
+# GreenMail
 
 To facilitate the configuration of Bileto in development, we use [GreenMail](https://greenmail-mail-test.github.io/greenmail/).
 

docs/developers/roles.md

@@ -1,4 +1,4 @@
-# Working with the roles & permissions
+# Roles & permissions
 
 In Bileto, the permissions are handled by the [Role](/src/Entity/Role.php) entity.
 Roles are associated to the users via the [Authorization](/src/Entity/Authorization.php) entity.

docs/developers/search-engine.md

@@ -1,4 +1,4 @@
-# Working with the search engine
+# Search engine
 
 The search engine is made of several parts:
 

docs/developers/sorters.md

@@ -1,4 +1,4 @@
-# Sort the entities
+# Sorting the entities
 
 As you display entities in the interface, you’ll probably want to sort them by some fields.
 Most likely you will want to sort them by names or titles.

docs/developers/translations.md

@@ -1,4 +1,4 @@
-# Using the translations
+# Translating Bileto
 
 Bileto uses the default system of translations of Symfony.
 Documentation: