Add a WebFlux vs. Servlets Security Section to the docs

[asaikali]

Add a section to the Introduction part of the docs about Reactive vs. Servlet security. This section should explain to users what are the fundamental differences between security in a reactive world and a servlet world. topics to touch on in the section are.

• Thread locals variables, how spring security uses them.
• Architecture assumptions spring security made that were no longer true in a reactive world and how did spring security adapt to the reactive requirements
• Background that users should know about how reactive applications execute code, such as schedulers in reactor ... etc.
• Does the reactive security support only apply to project reactor or can it be used with other reactive frameworks like RxJava
• Is there feature parity between reactive and servlet features of spring security, what the things that can't be done in reactive world or a servlet world.

[rwinch]

We could potentially do WebFlux vs Servlet, but Reactive vs Servlet is not really a good comparison since, for example, RSocket is only supported on the Reactive side and has no "Servlet" comparison

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[asaikali]

I have updated the title of the issue to be WebFlux vs. Servlet. In the ticket above Reactive and WebFlux were being used interchangeably.