Update URLs that are no longer accesible, or redirect to new locations.

[MorganR]

This updates URLs on the API docs page that are either:

• completely broken, or:
• redirect to a new location.

These are mostly simple changes (eg. minor subdomain changes or upgrades from http to https). However, there are a few that I'm less confident of, which you may want to double-check:

• https://ccmc.gsfc.nasa.gov/donki/ -> https://ccmc.gsfc.nasa.gov/tools/DONKI/
• http://eonet.sci.gsfc.nasa.gov/docs/v2.1 -> https://eonet.gsfc.nasa.gov/docs/v3 (for this one, the old link doesn't work, but the updated link could either point to docs for v2.1 or for v3. I'm not sure which is relevant to the API discussed here).
• https://spaceflight.nasa.gov/realdata/sightings/SSapplications/Post/JavaSSOP/SSOP_Help/tle_def.html -> https://celestrak.org/NORAD/documentation/tle-fmt.php (the old link results in "Access denied," and I found the new one via a web search).