Update Hecke banner

[paemurru]

I only recently found out that Hecke has its own manual. I included the manual link in the banner, similarly to the Julia and OSCAR banners.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 12 lines in your changes missing coverage. Please review.

Project coverage is 75.97%. Comparing base (8564bb0) to head (6b2f6ee).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Hecke.jl	0.00%	12 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #1675       +/-   ##
===========================================
+ Coverage    6.54%   75.97%   +69.42%     
===========================================
  Files         361      361               
  Lines      113587   113876      +289     
===========================================
+ Hits         7438    86515    +79077     
+ Misses     106149    27361    -78788     

Files with missing lines	Coverage Δ
src/Hecke.jl	51.11% <0.00%> (+8.72%)	⬆️

... and 320 files with indirect coverage changes

[thofma]

I did not count, but does this fit a terminal with 80 characters?

[paemurru]

Yes, it is 73 characters wide