Fix outdated docs

[jsheunis]

Fixes:

• demo site link in docs
• outdated tutorial link in readme
• outdated command syntax in readme

Closes #428

[netlify]

✅ Deploy Preview for datalad-catalog canceled.

Name	Link
🔨 Latest commit	f0a8660
🔍 Latest deploy log	https://app.netlify.com/sites/datalad-catalog/deploys/661da09974cf8900080ec1e4

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.59%. Comparing base (44ab1da) to head (f0a8660).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #437      +/-   ##
==========================================
+ Coverage   82.12%   84.59%   +2.46%     
==========================================
  Files          46       43       -3     
  Lines        2999     2862     -137     
==========================================
- Hits         2463     2421      -42     
+ Misses        536      441      -95     

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