DOCS: add dhcid RR docs

[fritterhoff]

Fixes #2695

[cafferata]

Thank you for this work! Can you also update the dnscontrol.d.ts? You can do this by running go generate ./....

[fritterhoff]

Done

[cafferata]

Thank you @fritterhoff. I see there is one more change needed:

SUMAMRY.md

    * [CAA_BUILDER](functions/domain/CAA_BUILDER.md)
    * [CNAME](functions/domain/CNAME.md)
+   * [DHCID](functions/domain/DHCID.md)
    * [DISABLE_IGNORE_SAFETY_CHECK](functions/domain/DISABLE_IGNORE_SAFETY_CHECK.md)
    * [DMARC_BUILDER](functions/domain/DMARC_BUILDER.md)
    * [DS](functions/domain/DS.md)
    * ```

[fritterhoff]

Also done ;-)

[cafferata]

Thank you @fritterhoff. See the online result here: 🚀
https://docs.dnscontrol.org/~/revisions/F27KYc5cDmhwljNT9zNF/language-reference/domain-modifiers/dhcid

[tlimoncelli]

Awesome! Thanks for following up on all this!