Cmake error fix clean (#36882)

### Details:
The master branch now requires CMake 3.26 at configure time, but docs
publishing environments can still expose an older system CMake such as
3.18.0. The release branch continues to build there because it has not
raised the top-level CMake minimum.

Add CMake to the documentation requirements so dependency installation
provides a compatible cmake executable before the docs CMake configure
step runs.

### Tickets:
- no ticket

### AI Assistance:
- AI assistance used: no

Author: michalzx

docs/requirements.txt

@@ -5,6 +5,7 @@ Babel==2.18.0
 beautifulsoup4==4.15.0
 breathe==4.36.0
 certifi==2026.6.17
+cmake>=3.26
 colorama==0.4.6
 Cython==3.2.8
 docutils==0.21.2