docs: add documentation for account e2ee setup at signup

## User Account creation

- This plugin creates an endpoint `/apps/ecloud-accounts/api/set_account_data` that is to be used to set user's email, quota,recovery email and create the user's folder if necessary

- Form for user account creation accessible from `/apps/ecloud-accounts/accounts/{lang}/signup`

- For development info on account creation, see [account creation docs](docs/account-creation.md)

### Captcha Configuration for user account creation

  - `ecloud-accounts.hcaptcha_site_key`

  - `ecloud-accounts.hcaptcha_secret`

### Passwords E2EE setup at signup configuration (disabled by default)

- If you want to enforce passwords E2EE setup at signup using the account password set by user:

  `occ config:app:set ecloud-accounts setupPasswordsE2eeAtSignup --value true --type=boolean`

- To disable:

  `occ config:app:set ecloud-accounts setupPasswordsE2eeAtSignup --value false --type=boolean`

## Drop account

- The drop account functionality plugin works in conjunction with the drop_account plugin : https://apps.nextcloud.com/apps/drop_account

### Actions done when the create method is triggered

- **Note: Steps that fail with an error are in bold; other steps fail silently in the background**

### When passwords E2EE setup at signup is disabled

```mermaid

flowchart TD

  K{Browser} --> L{Server}

  L --> A[<b>1. Validation</b>]

  A --> B[<b>2. Creation of LDAP record</b>]

  B --> C[3. Add username to permanent common data store]

  C --> D[<b>4. Set user data locally at nextcloud</b>]

  D --> E[5. Create HME and default @murena.io aliases]

  E --> F[<b>6. Set user ToS as accepted</b>]

  F --> G[<b>7. Set user language</b>]

  G --> H[8. Set Newsletter signup choice of user]

  H --> I[9. Set recovery email of user and trigger verification email]

  I --> J[10. Trigger welcome email]

```

### When passwords E2EE setup at signup is enabled

```mermaid

flowchart TD

  K{Browser} --> L{Server}

  L --> A[<b>1. Validation</b>]

  A --> B[<b>2. Creation of LDAP record</b>]

  B --> C[3. Add username to permanent common data store]

  C --> D[<b>4. Set user data locally at nextcloud</b>]

  D --> E[5. Create HME and default @murena.io aliases]

  E --> F[<b>6. Set user ToS as accepted</b>]

  F --> G[<b>7. Set user language</b>]

  G --> H[8. Set Newsletter signup choice of user]

  H --> I[9. Set recovery email of user and trigger verification email]

  I --> J[10. Trigger welcome email]

  J --> |Success| K

  K --> N[12. Compute and setup user encryption challenge at server via API call]

  N --> O[13. Setup user keychain at server via API call]

```