Add more guidance to web security optional exercises

[alodahl]

• This section offers a few links, but no clues on how to use them to experience basic web security practices. Add some directions for each link to help absolute beginners (who have read the previous outline, and know nothing else about the topic) know what to look for and what to try. Outside resources are ok as long as apprentices know exactly which parts to read, or skip, or do differently, etc.
  https://github.com/Techtonica/curriculum/blob/2e662db3eff0ea4c2db582a44e279ea269dcf803/security/web-security.md#extensions
• Please make supplemental resources and this further reading section sub-sections of "Extensions".

Also, if you have any opinions about the rest of the outline, please share.

Created by #1064

[stale]

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