Remove Zarr chunk manifest writing functionality

[norlandrhagen]

Aims to close #359. I included this in the zarr_reader PR, but since that's still a WIP, this separates it.

• Closes #xxxx
• Changes are documented in docs/releases.rst
• New functions/methods are listed in api.rst

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.15%. Comparing base (49b0e9d) to head (6bf2988).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #426      +/-   ##
==========================================
- Coverage   89.23%   89.15%   -0.08%     
==========================================
  Files          31       28       -3     
  Lines        1839     1706     -133     
==========================================
- Hits         1641     1521     -120     
+ Misses        198      185      -13     

Files with missing lines	Coverage Δ
virtualizarr/accessor.py	94.23% <ø> (-0.32%)	⬇️
virtualizarr/backend.py	97.01% <ø> (-0.09%)	⬇️
virtualizarr/readers/__init__.py	100.00% <ø> (ø)

... and 2 files with indirect coverage changes

[TomNicholas]

Thanks @norlandrhagen ! Just some minor nits then please merge!

FYI @abarciauskas-bgse

[TomNicholas]

(docs failing because warnings become errors)