Added Controllers

[blacksector]

I added four new controllers with these endpoints:

User Controller:

• GET /user/ ➡️ important, this is useful to check if a username is unique. If /user/omar returns 404 that means the username is available
• GET /user
• PATCH /user
• DELETE /user

Meeting Controller:

• GET /meetings
• POST /meeting

Appointment Controller:

• GET /appointments
• POST /appointment

Availability Controller:

• GET /availabilities
• POST /availability

Check out branch feature-api-docs for full documentation!