Update Storing Access Credentials section to improve readability

[kabilar]

• Convert instructions into a list
• Add headings for each subsection of the list
• Remove the code block syntax around the export DANDI_API_KEY=personal-key-value command since it does not render correctly when indented twice. The proposed syntax renders the bash command correctly:
  
• Reorder keyring instructions

[yarikoptic]

looks good to me, but let's also run by @jwodder since he would know better the auth procedure so double-checking would be great.

[yarikoptic]

it was approved. seems good to me -- let's proceed!