docs: add Agnik7 as a contributor for doc

[allcontributors]

Adds @Agnik7 as a contributor for doc.

This was requested by brosaplanella in this comment

[skip ci]

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (83d32ca) 99.58% compared to head (5f71273) 99.58%.
Report is 5 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #3399    +/-   ##
=========================================
  Coverage    99.58%   99.58%            
=========================================
  Files          254      256     +2     
  Lines        19821    19998   +177     
=========================================
+ Hits         19738    19915   +177     
  Misses          83       83            

Files	Coverage Δ
pybamm/expression_tree/averages.py	100.00% <ø> (ø)
...input/parameters/lithium_ion/Chen2020_composite.py	100.00% <ø> (ø)
...ameters/lithium_ion/Ecker2015_graphite_halfcell.py	100.00% <100.00%> (ø)
...rs/lithium_ion/OKane2022_graphite_SiOx_halfcell.py	100.00% <100.00%> (ø)
pybamm/input/parameters/lithium_ion/Xu2019.py	100.00% <ø> (ø)
...l_battery_models/lead_acid/base_lead_acid_model.py	100.00% <100.00%> (ø)
...models/full_battery_models/lithium_ion/Yang2017.py	100.00% <ø> (ø)
...ttery_models/lithium_ion/base_lithium_ion_model.py	100.00% <100.00%> (ø)
..._battery_models/lithium_ion/basic_dfn_half_cell.py	100.00% <100.00%> (+1.11%)	⬆️
...tery_models/lithium_ion/electrode_soh_half_cell.py	100.00% <100.00%> (ø)
... and 21 more

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