doc: add a pdf doc link in overview.rst

Author: fanghuaqi

doc/source/overview/overview.rst

@@ -25,6 +25,8 @@ In this new verison, we have following updates.
 - Integrate the APB interface peripherals(GPIO, I2C, UART, SPI, PWM) from `PULP Platform <https://github.com/pulp-platform>`__ into Hummingbirdv2 SoC, these peripherals are implemented in Verilog language, so it's easy for user to understand.
 - Add new development boards(Nuclei ddr200t and mcu200t) support for Hummingbirdv2 SoC.
 
+You can find a pdf version of this doc from https://doc.nucleisys.com/hbirdv2/hbirdv2_soc.pdf
+
 **Welcome to visit** `HBird SDK <https://github.com/riscv-mcu/hbird-sdk/>`__ **to use software development kit for the Hummingbirdv2 E203.**
 
 **Welcome to visit** `Hummingbird Forum <https://www.rvmcu.com/community-community.html>`__ **to participate in the discussion of the Hummingbirdv2 E203.**