🐛 FIX encoded URL ID references

[chrisjsewell]

For example <#name with spaces> will be encoded as #name%20with%20spaces, so we need to decode before resolving the reference.

[codecov]

Codecov Report

Base: 89.36% // Head: 89.40% // Increases project coverage by +0.03% 🎉

Coverage data is based on head (b75636b) compared to base (4cac732).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #715      +/-   ##
==========================================
+ Coverage   89.36%   89.40%   +0.03%     
==========================================
  Files          23       23              
  Lines        2867     2868       +1     
==========================================
+ Hits         2562     2564       +2     
+ Misses        305      304       -1     

Flag	Coverage Δ
pytests	89.40% <100.00%> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
myst_parser/mdit_to_docutils/base.py	92.88% <100.00%> (ø)
myst_parser/mdit_to_docutils/transforms.py	86.84% <100.00%> (+1.50%)	⬆️

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