Documentation/netutils: add comprehensive guide for tftpc - #20493

Summary

This PR replaces the 5-line placeholder stub in Documentation/applications/netutils/tftpc/index.rst with a comprehensive, production-grade guide for the tftpc (TFTP client) network utility.

Addresses #11081

Key Additions:

• Architecture Overview: Documents tftpc's dual role as both the interactive NSH get and put shell commands engine and a standalone programmatic C API.
• Core Capabilities: Covers RFC 1350 compliance, binary (octet) and text (netascii) transfer modes, streaming callback architecture, dynamic TID port negotiation, and automatic retransmission timeouts.
• Security Considerations: Details caveats for firmware transfers over TFTP, emphasizing isolated network environments and the necessity of independent cryptographic verification (verified digital signatures or out-of-band digests).
• Kconfig Options & Dependencies: Outlines prerequisite networking subsystems (CONFIG_NET_UDP, CONFIG_NET_IPv4) and all tuneable options (CONFIG_NETUTILS_TFTPC, CONFIG_NETUTILS_TFTP_PORT, CONFIG_NETUTILS_TFTP_TIMEOUT, CONFIG_NETUTILS_TFTP_DUMPBUFFERS).
• NSH CLI Guide: Full command synopses, option flags (-b, -n, -f, -h), and practical shell examples for both get and put.
• C API Reference: Detailed coverage of filesystem helpers (tftpget(), tftpput()) and streaming callback APIs (tftpget_cb(), tftpput_cb()).
• Production C Application Example: Provides a runnable embedded file-download helper demonstrating memory sink callbacks with buffer boundary checks and proper errno returns.

Impact

• Documentation: Fills another critical unwritten documentation page tracked in Documentation: blank pages in documentation #11081.
• User Experience: Provides clear, secure patterns for embedded firmware transfer and shell usage in NuttX.
• Code/Build: Documentation-only change. No functional code or ABI alterations.

Testing

• Validated Sphinx reStructuredText syntax, formatting, and header underline lengths.
• Verified NSH CLI flags against apps/nshlib/nsh_command.c and nsh_netcmds.c.
• Verified C API definitions and callback signatures against apps/include/netutils/tftp.h.