[doc] Fix GUIDE.md build instructions and add more info on OT logs.

[EskoDijk]

[codecov-commenter]

Codecov Report

Merging #351 (419b4bc) into main (c616148) will decrease coverage by 0.04%. The diff coverage is 50.00%.

@@            Coverage Diff             @@
##             main     #351      +/-   ##
==========================================
- Coverage   49.97%   49.93%   -0.05%     
==========================================
  Files          38       38              
  Lines        4608     4604       -4     
==========================================
- Hits         2303     2299       -4     
  Misses       2123     2123              
  Partials      182      182              

Impacted Files	Coverage Δ
dispatcher/dispatcher.go	57.10% <0.00%> (ø)
web/site/bindata.go	99.00% <ø> (-0.01%)	:arrow_down:
otnstester/OtnsTest.go	89.44% <100.00%> (ø)