docs: add documentation for nft module

[julienrbrt]

Description

Follow-up of #13836 for docs.

---

Author Checklist

All items are required. Please add a note to the item if the item is not applicable and
please add links to any relevant follow up issues.

I have...

• included the correct type prefix in the PR title
• added ! to the type prefix if API or client breaking change
• targeted the correct branch (see PR Targeting)
• provided a link to the relevant issue or specification
• followed the guidelines for building modules
• included the necessary unit and integration tests
• added a changelog entry to CHANGELOG.md
• included comments for documenting Go code
• updated the relevant documentation or specification
• reviewed "Files changed" and left comments if necessary
• confirmed all CI checks have passed

Reviewers Checklist

All items are required. Please add a note if the item is not applicable and please add
your handle next to the items reviewed if you only reviewed selected items.

I have...

• confirmed the correct type prefix in the PR title
• confirmed ! in the type prefix if API or client breaking change
• confirmed all author checklist items have been addressed
• reviewed state machine logic
• reviewed API design and naming
• reviewed documentation is accurate
• reviewed tests and test coverage
• manually tested (if applicable)

[codecov]

Codecov Report

Merging #13871 (3b2daef) into main (5f4350d) will decrease coverage by 0.15%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13871      +/-   ##
==========================================
- Coverage   56.69%   56.53%   -0.16%     
==========================================
  Files         642      638       -4     
  Lines       55136    54877     -259     
==========================================
- Hits        31258    31024     -234     
+ Misses      21347    21333      -14     
+ Partials     2531     2520      -11     

Impacted Files	Coverage Δ
x/distribution/simulation/operations.go	80.64% <0.00%> (-9.68%)	⬇️
errors/handle.go
errors/errors.go
errors/abci.go
errors/stacktrace.go