Add comprehensive docs on file integration

[niveathika]

1. File Integration in WSO2 Integrator: BI

   • Entry point to file integration

2. [Listener] Processing Local Files - Current

   service "localObserver" on inFolder {
   
   remote function onCreate(file:FileEvent m) {
       string msg = "Create: " + m.name;
       log:printInfo(msg);
   }
   }

3. [Listener] Work with remote servers

   1. Setup for FTP, FTPS, SFTP

4. Full fledged file integration usecase,

   1. Listen to directory
   2. Process new CSV files,
   3. Validate a field in the CSV record.
   4. If 90% passes

5. [Client] Writing Files
   HTTP service which will trigger a file send

6. Handling large files

   1. Streaming is your friend
   2. Process a 10000 record CSV?

7. Troubleshooting Guide

   1. Scenario: Connection Refused (FTP/SFTP)
      1. Firewall/network
      2. Debugging-> Connect through a client and check?
   2. Authentication failed
      1. Verify password
      2. Verify SSH permission
      3. Failing in mac with default private key -> BUG
   3. Certificate is untrusted
      1. Add cert
      2. If refused? what is the error message
   4. Permission for the folder is refused
   5. Enable debug logs easily

[niveathika]

The docs are fleshed out, and 4 end-to-end scenarios are done. However, this cannot proceed until BI File integration flow improvements are completed (wso2/product-ballerina-integrator#1684).