docs: add docs for flows, start docs for stages

docs/flow/flows.md

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+# Flows
+
+Flows are a method of describing a sequence of stages. A stage represents a single verification or logic step. They are used to authenticate users, enroll them, and more.
+
+Upon Flow execution, a plan is generated, which contains all stages. This means upon execution, all attached policies are evaluated. This behaviour can be altered by enabling the `Re-evaluate Policies` option on the binding.
+
+To determine which flow is linked, passbook searches all Flows with the required designation and chooses the first instance the current user has access to.
+
+## Permissions
+
+Flows can have policies assigned to them, which determines if the current user is allowed to see and use this flow.
+
+## Designation
+
+Flows are designated for a single Purpose. This designation changes when a Flow is used. The following designations are available:
+
+### Authentication
+
+This is designates a flow to be used for authentication.
+
+The authentication flow should always contain a `user_login` stage, which attaches the staged user to the current session.
+
+### Invalidation
+
+This designates a flow to be used for the invalidation of a session.
+
+This stage should always contain a `user_logout` stage, which resets the current session.
+
+### Enrollment
+
+This designates a flow for enrollment. This flow can contain any amount of Prompt stages, E-Mail verification or Captchas. At the end to create the user, you can use the `user_write` stage, which either updates the currently staged user, or if none exists, creates a new one.
+
+### Unenrollment
+
+This designates a flow for unenrollment. This flow can contain any amount of verification, like `email` or captcha. To finally delete the account, use the `user_delete` stage.
+
+### Recovery
+
+This designates a flow for recovery. This flow normally contains an `identification` stage to find the user. Then it can contain any amount of verification, like `email` or captcha.
+Afterwards, use the `prompt` stage to ask the user for a new password and use `user_write` to update the password.
+
+### Change Password
+
+This designates a flow for password changing. This flow can contain any amount of verification, like `email` or captcha.
+Afterwards, use the `prompt` stage to ask the user for a new password and use `user_write` to update the password.

docs/flow/stages/captcha/captcha.md

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+# Captcha stage
+
+This stage adds a form of verification using [Google's ReCaptcha](https://www.google.com/recaptcha/intro/v3.html).
+
+This stage has two required fields. You need a Public and a Private key, both of which you can acquire at https://www.google.com/recaptcha/admin.
+
+![](captcha-admin.png)

docs/flow/stages/dummy/dummy.md

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+# Dummy stage
+
+This stage is used for development, and has no function. It presents the User with a form, that requires a single confirmation.
+
+![](dummy.png)