fix: BlogPage may only be created under BlogIndexPage

[mgax]

Fix #337.
Fix #271.

The blog page template depends on its parent page being a BlogPageIndex, so it crashes if its parent is anything else.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.73%. Comparing base (4ad3143) to head (336946f).
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #401      +/-   ##
==========================================
+ Coverage   82.72%   82.73%   +0.01%     
==========================================
  Files         124      124              
  Lines        2599     2601       +2     
==========================================
+ Hits         2150     2152       +2     
  Misses        449      449              

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