Include links to required readings and permuted index in Lessons

[Jessie Dotson]

The Lessons have so much good info in them -- super useful! At the beginning of the lessons, users are referred to spice required readings and the permuted index. The text of the lessons tells you where to find this info in the C spice toolkit, which makes one think you have to download the C toolkit in order to follow along with the spiceypy lessons... But, NAIF, does have this info online -- so, no download is required. It would lower the barrier to new users if the Lessons included links to these key resources at NAIF (e.g. https://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/req/index.html, https://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/info/cspice_idx.html)