Land #18428, Add mssql_login docs

This PR adds a documentation file for the mssql_login scanner.

documentation/modules/auxiliary/scanner/mssql/mssql_login.md

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+## Vulnerable Application
+
+This module simply queries the MSSQL instance for a specific user/pass (default is sa with blank).
+
+### Setup
+A docker container can be spun up with the following command to test this module:
+`docker run -e 'ACCEPT_EULA=Y' -e 'MSSQL_SA_PASSWORD=N0tpassword!' -p 1433:1433 -d mcr.microsoft.com/mssql/server:2022-latest`
+
+## Verification Steps
+1. Start msfconsole
+2. Do: `use scanner/mssql/mssql_login`
+3. Do: `set RHOSTS [IP]`
+4. Do: `run`
+5. You should get a shell.
+
+## Options
+
+### USER_FILE
+
+File containing users, one per line.
+
+### PASS_FILE
+
+File containing passwords, one per line
+
+## Scenarios
+```
+msf > use scanner/mssql/mssql_login
+msf6 auxiliary(scanner/mssql/mssql_login) > set rhosts 127.0.0.1
+rhosts => 127.0.0.1
+msf6 auxiliary(scanner/mssql/mssql_login) > set password N0tpassword!
+password => N0tpassword!
+msf6 auxiliary(scanner/mssql/mssql_login) > options
+
+Module options (auxiliary/scanner/mssql/mssql_login):
+
+   Name                 Current Setting  Required  Description
+   ----                 ---------------  --------  -----------
+   ANONYMOUS_LOGIN      false            yes       Attempt to login with a blank username and password
+   BLANK_PASSWORDS      true             no        Try blank passwords for all users
+   BRUTEFORCE_SPEED     5                yes       How fast to bruteforce, from 0 to 5
+   DB_ALL_CREDS         false            no        Try each user/password couple stored in the current database
+   DB_ALL_PASS          false            no        Add all passwords in the current database to the list
+   DB_ALL_USERS         false            no        Add all users in the current database to the list
+   DB_SKIP_EXISTING     none             no        Skip existing credentials stored in the current database (Accepted: none, user, user&realm)
+   PASSWORD             N0tpassword!     no        A specific password to authenticate with
+   PASS_FILE                             no        File containing passwords, one per line
+   RHOSTS               127.0.0.1        yes       The target host(s), see https://docs.metasploit.com/docs/using-metasploit/basics/using-metasploit.html
+   RPORT                1433             yes       The target port (TCP)
+   STOP_ON_SUCCESS      false            yes       Stop guessing when a credential works for a host
+   TDSENCRYPTION        false            yes       Use TLS/SSL for TDS data "Force Encryption"
+   THREADS              1                yes       The number of concurrent threads (max one per host)
+   USERNAME             sa               no        A specific username to authenticate as
+   USERPASS_FILE                         no        File containing users and passwords separated by space, one pair per line
+   USER_AS_PASS         false            no        Try the username as the password for all users
+   USER_FILE                             no        File containing usernames, one per line
+   USE_WINDOWS_AUTHENT  false            yes       Use windows authentication (requires DOMAIN option set)
+   VERBOSE              true             yes       Whether to print output for all attempts
+
+
+View the full module info with the info, or info -d command.
+
+msf6 auxiliary(scanner/mssql/mssql_login) > run
+
+[*] 127.0.0.1:1433        - 127.0.0.1:1433 - MSSQL - Starting authentication scanner.
+[!] 127.0.0.1:1433        - No active DB -- Credential data will not be saved!
+[+] 127.0.0.1:1433        - 127.0.0.1:1433 - Login Successful: WORKSTATION\sa:N0tpassword!
+[*] 127.0.0.1:1433        - Scanned 1 of 1 hosts (100% complete)
+[*] Auxiliary module execution completed
+msf6 auxiliary(scanner/mssql/mssql_login) >
+```