add link to JSON interface spec docs

[cyborgRen]

Description

Please include a summary of the changes and be sure to follow our Contribution Guidelines.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• I have selected the correct base branch.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• Any dependent changes have been merged and published in downstream modules.
• I ran npm run lint with success and extended the tests and types if necessary.
• I ran npm run test:unit with success.
• I ran npm run test:coverage and my test cases cover all the lines and branches of the added code.
• I ran npm run build and tested dist/web3.min.js in a browser.
• I have tested my code on the live network.
• I have checked the Deploy Preview and it looks correct.
• I have updated the CHANGELOG.md file in the root folder.
• I have linked Issue(s) with this PR in "Linked Issues" menu.

[codecov]

Codecov Report

Merging #6832 (7960dea) into 4.x (4358140) will increase coverage by 4.66%.
Report is 181 commits behind head on 4.x.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##              4.x    #6832      +/-   ##
==========================================
+ Coverage   87.37%   92.04%   +4.66%     
==========================================
  Files         197      214      +17     
  Lines        7548     8192     +644     
  Branches     2059     2206     +147     
==========================================
+ Hits         6595     7540     +945     
+ Misses        953      652     -301     

Flag	Coverage Δ
UnitTests	92.04% <ø> (+4.66%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
web3	∅ <ø> (∅)
web3-core	∅ <ø> (∅)
web3-errors	∅ <ø> (∅)
web3-eth	∅ <ø> (∅)
web3-eth-abi	∅ <ø> (∅)
web3-eth-accounts	∅ <ø> (∅)
web3-eth-contract	∅ <ø> (∅)
web3-eth-ens	∅ <ø> (∅)
web3-eth-iban	∅ <ø> (∅)
web3-eth-personal	∅ <ø> (∅)
web3-net	∅ <ø> (∅)
web3-providers-http	∅ <ø> (∅)
web3-providers-ipc	∅ <ø> (∅)
web3-providers-ws	∅ <ø> (∅)
web3-rpc-methods	∅ <ø> (∅)
web3-utils	∅ <ø> (∅)
web3-validator	∅ <ø> (∅)