Add more navigation headers for file format details

Make it easier to find key formats and, more importantly because it's
often missed, date-time recommendations.

Signed-off-by: Joshua Lock <[email protected]>

Author: joshuagl

tuf-spec.md

@@ -545,8 +545,9 @@ All signed metadata objects have the format:
       ::
         A hex-encoded signature of the canonical form of the metadata for <a for="role">ROLE</a>.
 
+## File formats: keys ## {#file-formats-keys}
 
-All <dfn>KEY</dfn>s have the format:
+All <dfn>KEY</dfn> objects have the format:
 
 <pre highlight="json">
 {
@@ -654,6 +655,8 @@ The <dfn for="keytype">"ecdsa"</dfn> format is:
   ::
     PEM format and a string.
 
+## File formats: date-time ## {#file-formats-date-time}
+
 Metadata <dfn>date-time</dfn> follows the ISO 8601 standard.  The expected
 format of the combined date and time string is "YYYY-MM-DDTHH:MM:SSZ".  Time is
 always in UTC, and the "Z" time zone designator is attached to indicate a