doc: libconsensus: add missing error code description, fix NBitcoin link

[theStack]

This PR improves the libbitcoinconsensus description in shared-libraries.md in two ways:

• adds the missing error code description for bitcoinconsensus_ERR_INVALID_FLAGS (introduced by commit 5ca8ef2, PR libconsensus: Add input validation of flags #8976)
• updates and fixes the link to the NBitcoin implementation (introduced by commit 3361edd, PR doc: add documentation for shared library libbitcoinconsensus #6430)
  • the owner of the NBitcoin github repository changed from NicolasDorier to MetacoSA (redirection still worked though)
  • instead of dynamically referring to a file in master with a fixed line number (which is obviously always quickly outdated), use a permalink with a file numbers area

[DrahtBot]

🕵️ @harding has been requested to review this pull request as specified in the REVIEWERS file.

[maflcko]

cr ACK cb0b712

[harding]

Code (documentation) review ACK cb0b712. Text is clear and seems accurate, and the link checks out.