Fix broken links

• “User Tutorials” link under user manual -> overview (here) is dead
• There’s a misdirected link for ‘environmental data standards’. It takes you way back up to the ‘environmental conditions’ page under data products. Because of this, if you’re clicking through in order, you get hung up there when it redirects you way back up to the top. @craig-willis - I don't see an "environmental data standards" page to link to
• What is the point of having the downwelling spectral irradiance tab under data product creation? What do you want to put in there from the github post thread, which appears to now be closed as of 11/30? @craig-willis
• Under ‘hyperspectral data’, it says there are 3 software dependencies. Only 2 are listed – one of them has a whole bunch of stuff there including some French, the other is a dead link. Also, this page should include instructions on how to install whatever dependencies are needed. Also, for some reinforcement, I would explain what data level is output by the terraref.sh script when it converts to netCDF.
• Finally, there are some placeholders in the intro paragraph on this page.

[craig-willis]

I've removed these from the TOC and the empty page -- these were likely originally intended for someone else to fill in. I've done nothing with the hyperspectral pages.