Clarify bike path specification

[gregmacfarlane]

In docs/spec/segment, the National Bikeway Network Data Template no longer exists at the ORNL website, and a google search does not readily return that table. Should define a table in the specification

[ssmith55]

Thanks for pointing out that dead link. An updated link is in the Template in https://data.transportation.gov/stories/s/National-Bicycle-Network/88zh-3rqb/ Look for Table 1- A. Also, the values are enumerated in the json version of the spec. https://github.com/zephyr-data-specs/GMNS/blob/main/spec/link.schema.json "categories": [
"unseparated bike lane",
"buffered bike lane",
"separated bike lane",
"counter-flow bike lane",
"paved shoulder",
"shared lane",
"shared use path",
"off-road unpaved trail",
"other",
"none"