Improve issue templates

[ricardoV94]

Made some changes to the template issues

Changes and motivation

1. No point in forcing a link to a discussion in the feature request issue. We can easily convert an issue to a discussion or vice-versa ourselves
2. Not requiring a link to existing documentation. If the user is asking for new documentation, it's not relevant. The less annoying the better I would say.
3. Remove installation issue template. I think it's fine to direct users to discourse for this? If it's actually a problem it would become a bug issue later
4. Add free-form issue for developers

Docs / Maintenance

• Improve template issues

[codecov]

Codecov Report

Merging #6345 (f30be1f) into main (3ff4e7a) will decrease coverage by 3.29%.
The diff coverage is 100.00%.

❗ Current head f30be1f differs from pull request most recent head 9e88e26. Consider uploading reports for the commit 9e88e26 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6345      +/-   ##
==========================================
- Coverage   94.15%   90.85%   -3.30%     
==========================================
  Files         132      132              
  Lines       26726    26730       +4     
==========================================
- Hits        25163    24285     -878     
- Misses       1563     2445     +882     

Impacted Files	Coverage Δ
pymc/variational/opvi.py	75.27% <ø> (-11.81%)	⬇️
pymc/model.py	89.84% <100.00%> (-0.09%)	⬇️
pymc/tests/variational/test_opvi.py	0.00% <0.00%> (-100.00%)	⬇️
pymc/tests/variational/test_callbacks.py	0.00% <0.00%> (-100.00%)	⬇️
pymc/tests/variational/test_approximations.py	0.00% <0.00%> (-100.00%)	⬇️
pymc/tests/variational/test_inference.py	0.00% <0.00%> (-99.03%)	⬇️
pymc/variational/test_functions.py	38.09% <0.00%> (-61.91%)	⬇️
pymc/variational/stein.py	48.33% <0.00%> (-51.67%)	⬇️
pymc/data.py	36.17% <0.00%> (-43.91%)	⬇️
pymc/variational/inference.py	50.77% <0.00%> (-34.20%)	⬇️
... and 11 more