Update EIP-721: Add ERC links

[fulldecent]

This fixes some linter complaints

Here is a validation run that reporting the error which this PR fixes.

https://github.com/ethereum/EIPs/actions/runs/6045138001/job/16404727259?pr=6527

[eth-bot]

File EIPS/eip-721.md

Requires 1 more reviewers from @axic, @gcolvin, @lightclient, @SamWilsn

[Pandapip1]

ERC-721 should be replaced with "This EIP" instead of self-referencing by number.

Library links aren't supported. Use relative links instead (i.e. [ERC-20](./eip-20.md) instead of [ERC-20](eip-20).

[fulldecent]

@Pandapip1 Thank you for your notes. I have updated all references to "ERC-721" from within that document to "this ERC". Some of this required a little disambiguation because this document is referring to an ERC (numbered 721) an interface (named ERC721) and two other interfaces (ERC721Enumerable, ERC721Metadata). My action here was to maintain clarity about what was being discussed while minimizing word changes.

This document previously refers to these things as "ERC"s. I kept this nomenclature in the word changes above. If there will consideration of changing this to "EIP", please let's separate that to a different, independent discussion and PR.

A reference was previously made to 0xcert's implementation which is named ERC-721. I updated this to its new name "Nibbstack". But the new product is still named ERC721. Probably this will also trigger a false-positive lint.

[Pandapip1]

+1 if my change fixed the errors

[fulldecent]

@Pandapip1 thank you, yes that's good now!

[SamWilsn]

I undid a few content changes, and fixed a few more eipw errors. Still a bunch of them because of the external links.

I'll add this to tomorrow's EIPIP call agenda.

[github-actions]

The commit 7e2b87e (as a parent of aecf612) contains errors.
Please inspect the Run Summary for details.

[bumblefudge]

thanks for this

@lightclient will be reviewing

[SamWilsn]

I am closing this pull request because we are in the process of separating EIPs and ERCs into distinct repositories. Unfortunately, as far as we are aware, GitHub does not provide any tools to ease this migration, so every pull request will need to be re-opened manually.

As this is a PR to create / modify an ERC, I will kindly ask you to redirect this to the new repository at ethereum/ERCs. We have prepared a guide to help with the process.

If there is relevant history here, please link to this PR from the new pull request.

On behalf of the EIP Editors, I apologize for this inconvenience.