CI: rename 'gh-pages' branch to 'docs'

[lukaszstolarczuk]

We've begun the process of moving the gh-pages content - generated docs - into pmem/pmem.github.io repository. It's now required to remove gh-pages branch here because its content conflicts with pmem.github.io's content.

• [x] before merging this change we have to actually update the name of the branch (gh-pages -> docs)

---

This change is 

[codecov[bot]]

Codecov Report

Merging #1060 (3fb8321) into master (4c2663e) will decrease coverage by 0.02%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1060      +/-   ##
==========================================
- Coverage   92.72%   92.70%   -0.03%     
==========================================
  Files          38       38              
  Lines        3796     3796              
==========================================
- Hits         3520     3519       -1     
- Misses        276      277       +1     

Impacted Files	Coverage Δ
src/engines-experimental/robinhood.cc	90.64% <0.00%> (-0.33%)	:arrow_down: