[Doctrine] Reword the docs about Doctrine

[javiereguiluz]

This is a meta-issue to link to all the pull requests related to this:

• Reworded the intro of the article ([Doctrine] Reword the intro of the main article #12314)
• Merge all articles about "database + testing" ([Doctrine] Put all docs about testing databases in a single article #12317)
• Remove the "registration form" article (Removed the article about the registration form #12326)
• Merge all articles about "database + sessions" (Reworded the docs about storing sessions in database #12325)
• Merge all articles about "Doctrine events" (Reworded the docs about Doctrine events #12328)
• Moved the Doctrine Extensions to the main Doctrine article (Moved the Doctrine Extensions to the main Doctrine article #12332)

[javiereguiluz]

Closing as fixed because all the related PR are merged.