Docs for Credentials (XLS-70d)

[mDuo13]

Spec: https://github.com/XRPLF/XRPL-Standards/tree/master/XLS-0070d-credentials rippled PR: https://github.com/XRPLF/rippled/pull/5103

Updated with changes from https://github.com/XRPLF/XRPL-Standards/pull/232

[mDuo13]

Changes in the latest commits:

• Rebased on latest main
• Fixed the landing page to remove dependency on legacy Redocly portal UI components and use new Cards instead.
• Addressed reviewer feedback.
• Moved the common link definitions to a snippet with the same name/path as the one in the XRPL dev portal repo, to minimize the effort of porting files over when they get incorporated into an official release.

[mDuo13]

Closing and reopening to work around a bug in the CI system.