Add numbers to the headers of tutorials 7, 8, 9

[danielle-pinto]

• Add headers to tutorials 7, 8, 9
• +++
  using Dates
  date = Date("2026-01-02")
  title = "Problem 5: Computing GC Content"
  rss_descr = "Solving Rosalind problem GC — computing GC content from FASTA files — using base Julia and FASTX.jl."
  +++
• Kevin suggested adding an hfun to do this
  example is here: https://github.com/kescobo/audiommunity.org/blob/main/utils.jl#L293
• Fix the publishing date

[danielle-pinto]

Add numbers to cookbook tutorials as well