sys/net/app/cord: update doc

[Vjorald]

Contribution description

This PR updates the doc file of the CORD module and examples in RIOT. Updates:

• Update of the references to RFC 9176.
• Removal of the label Not Yet Implemented in the doc file of the CORD module, since it is obsolete.

Testing procedure

• The draft-ietf-core-resource-directory-27 was compared with the RFC 9176 and no necessary changes in the implementation were found.
• An experimental Lookup Client was built, to test if the functions from the cord_lc of the CORD module in RIOT were implemented. Lookup requests were sent to an aiocoap Resource Directory implementation. All the functions in the cord_lc returned the expected results.

Issues/PRs references

• Comparison draft-ietf-core-resource-directory-27 vs RFC 9176
• aiocoap Resource Directory
• RIOT Lookup Client

[chrysn]

Thanks for the update; let's clear the one comment and then this can go in.

[riot-ci]

Murdock results

✔️ PASSED

d2600f8 cord/doc: Remove the label (Not Yet Implemented) and the group part

Success	Failures	Total	Runtime
1	0	1	01m:17s

Artifacts

• Documentation preview

[chrysn]

Looks good, thanks. Please squash (either into one commit, or into the "update reference" and a "remove label" commit), and prefix them with "cord/doc:" or similar as per the project's convention.