doc: add instructions to build documentation

[thofma]

Closes #1642

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 75.84%. Comparing base (506e87c) to head (5dc3330).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Hecke.jl	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1645      +/-   ##
==========================================
- Coverage   75.84%   75.84%   -0.01%     
==========================================
  Files         361      361              
  Lines      113702   113702              
==========================================
- Hits        86241    86234       -7     
- Misses      27461    27468       +7     

Files with missing lines	Coverage Δ
src/Hecke.jl	51.77% <0.00%> (-0.17%)	⬇️

... and 29 files with indirect coverage changes