This commit adds extensive documentation for the ssl_keylog_file feature
(introduced in PR #4236), which enables TLS key logging for debugging
encrypted traffic.

## Background

The ssl_keylog_file variable (exposed as admin-ssl_keylog_file in SQL
interface) allows ProxySQL to write TLS secrets to a file in NSS Key Log
Format. These secrets can be used by tools like Wireshark and tshark to
decrypt and analyze TLS traffic for debugging purposes.

## Changes

### Inline Documentation (Code)

1. include/proxysql_sslkeylog.h (+96 lines)
   - File-level documentation explaining the module purpose and security
   - Doxygen comments for all 5 public APIs
   - Thread-safety annotations
   - Parameter descriptions and return values

2. lib/proxysql_sslkeylog.cpp (+136 lines)
   - Implementation-level documentation
   - Algorithm explanations (double-checked locking, thread safety)
   - Reference to NSS Key Log Format specification

3. include/proxysql_admin.h (+19 lines)
   - Variable documentation for ssl_keylog_file
   - Path handling rules (absolute vs relative)
   - Security implications

### Developer Documentation (doc/ssl_keylog/ssl_keylog_developer_guide.md)

Target audience: Developers working on ProxySQL codebase

Contents:
- Variable naming convention (SQL vs config file vs internal)
- Architecture diagrams
- Thread safety model (pthread rwlock)
- NSS Key Log Format specification
- Complete API reference for all public functions
- Integration points in the codebase
- Security considerations and code review checklist
- Testing procedures

### User Documentation (doc/ssl_keylog/ssl_keylog_user_guide.md)

Target audience: End users and system administrators

Contents:
- What is SSL key logging and when to use it
- Variable naming: admin-ssl_keylog_file (SQL) vs ssl_keylog_file (config)
- Step-by-step enable/disable instructions
- Path resolution (absolute vs relative)
- Log rotation procedures
- Production workflow: tcpdump capture → offline analysis
- Wireshark (GUI) integration tutorial
- tshark (command-line) usage examples
- Troubleshooting common issues
- Security best practices
- Quick reference card

## Key Features Documented

1. **Variable Naming Convention**
   - SQL interface: SET admin-ssl_keylog_file = '/path';
   - Config file: ssl_keylog_file='/path' (in admin_variables section)
   - Internal code: ssl_keylog_file

2. **Production Workflow**
   - Capture traffic with tcpdump (no GUI on production server)
   - Transfer pcap + keylog to analysis system
   - Analyze offline with Wireshark (GUI) or tshark (CLI)

3. **tshark Examples**
   - Command-line analysis of encrypted traffic
   - Filter examples for debugging TLS issues
   - JSON export for automated analysis

## Security Notes

The documentation emphasizes that:
- Key log files contain cryptographic secrets that decrypt ALL TLS traffic
- Access must be restricted (permissions 0600)
- Only enable for debugging, never in production
- Securely delete old key log files

## Files Modified
- include/proxysql_admin.h
- include/proxysql_sslkeylog.h
- lib/proxysql_sslkeylog.cpp

## Files Added
- doc/ssl_keylog/ssl_keylog_developer_guide.md
- doc/ssl_keylog/ssl_keylog_user_guide.md

This commit addresses all review comments from gemini-code-assist on PR #5279:

1. Fixed FLUSH LOGS documentation - clarified that file is reopened for
   appending, not truncating, and updated the note about preserving contents

2. Fixed callback documentation - clarified that the callback attaches to
   all frontend connections, not just admin connections

3. Updated security warning - focused on passive eavesdropping and offline
   decryption as the primary threats

4. Fixed typo: proxyql_ip -> proxysql_ip in tcpdump example

5. Removed misleading @see HPKP link - HPKP is unrelated to NSS Key Log
   Format and is a deprecated feature

6. Updated NSS Key Log Format URL to use official MDN link instead of
   unofficial mirror

7. Fixed buffer size comment to accurately reflect 256-byte buffer and
   254-byte line length validation

8. Clarified fputs comment to emphasize the read lock's role in allowing
   concurrent writes from multiple threads