✨ Warn if curating against gene symbols

[Zethson]

Fixes laminlabs/bionty#143

• Adds a warning if the var_index is bt.Gene.symbol to check out the FAQ for more details on why this is bad
• I decided against hard coding a URL because it can change.
• I decided against adding this warning in curate() because it would get triggered repeatedly which can be annoying

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.60%. Comparing base (57fbd29) to head (4624ae9).
Report is 14 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2142      +/-   ##
==========================================
+ Coverage   92.53%   92.60%   +0.06%     
==========================================
  Files          55       55              
  Lines        6467     6474       +7     
==========================================
+ Hits         5984     5995      +11     
+ Misses        483      479       -4     

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[github-actions]

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