Heading levels are messed up in the reference page of SecurityBundle

[stof]

https://symfony.com/doc/current/reference/configuration/security.html has a messed up heading hierarchy.

access_control and firewalls are top-level keys, not subkeys of session_fixation_strategy. And all headings coming after firewalls should be sub-heading of firewalls as they document its subkeys. role_hierarchy and providers are also top-level jets but are documented as subkeys of required_badges (which is should be one of the firewalls subkeys)

some other things also don't make sense at all. delete_cookies is a sub-key of logout but is not documented as such (fortunately, there is a code snippet).

[stof]

Note that the reference is also incomplete. For instance, the top-level key access_decision_manager does not appear anywhere in the page. And password_hashers does not appear either.

[javiereguiluz]

@OskarStark I'm removing myself from being assigned in this issue. I'm sorry but I don't understand the Symfony Security component well, so I can't work on this.

[mmauksch]

Fixing the headings was a quick fix.

In general that site might need some love in general though. I think moving some of the links mentioned in basic options to the relevant subheadings to give these links more context. My first day contributing to symfony docs though so I would prefer to read up a bit more on how these pages should be structured before I do any bigger changes.