Document fixNFTokenPageLinks amendment and associated LedgerStateFix transaction

[mDuo13]

Anyone can submit a transaction of the new type to fix a specific type of corruption in the ledger (missing links in NFTokenPage directories). In the future, the same transaction type may be extended to fix other types of state corruption as needed. (Currently, there are no other known types of corruption that need fixing.)

XRPLF/rippled#4945 has a pretty thorough writeup about the fix amendment and the solution. The PR has been merged to the develop branch and will probably be part of rippled 2.3.0.

The documentation is being addressed in two phases:

• Draft content when amendment is in development.
• Update xrpl.org once amendment is proposed in stable release.

[DennisDawson]

I have a write-up of this that's been sitting in Draft mode on opensource waiting for the feature to go live. ripple/opensource.ripple.com#78