fix(cmake): Add missing POST_BUILD option to travis_doc target

[jcfr]

This addresses the following warning associated with CMP0175 policy introduced in CMake 3.31:

CMake Warning (dev) at CMakeLists.txt:165 (add_custom_command):
  Exactly one of PRE_BUILD, PRE_LINK, or POST_BUILD must be given.  Assuming
  POST_BUILD to preserve backward compatibility.

  Policy CMP0175 is not set: add_custom_command() rejects invalid arguments.
  Run "cmake --help-policy CMP0175" for policy details.  Use the cmake_policy
  command to set the policy and suppress this warning.
This warning is for project developers.  Use -Wno-dev to suppress it.

[tencent-adm]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[jcfr]

Supersedes:

• CMakeLists: fix add_custom_command warning #2337