Updates and formats Building ISIS

[jrcain-usgs]

The Building ISIS doc seemed to be kind of outdated and contain some things that don't work anymore. I tried to streamline the first half at least, based on my recent builds.

For Example:

• -DJP2KFLAG is OFF by default
• Some old flags no longer work
• The install directory is never used, everything happens in the build directory
• Added a cmake flag example
• No one uses the shared internal environments/the setisis script under `For Internal Devs' anymore, right?

Licensing

This project is mostly composed of free and unencumbered software released into the public domain, and we are unlikely to accept contributions that are not also released into the public domain. Somewhere near the top of each file should have these words:

This work is free and unencumbered software released into the public domain. In jurisdictions that recognize copyright laws, the author or authors of this software dedicate any and all copyright interest in the software to the public domain.

• I dedicate any and all copyright interest in this software to the public domain. I make this dedication for the benefit of the public at large and to the detriment of my heirs and successors. I intend this dedication to be an overt act of relinquishment in perpetuity of all present and future rights to this software under copyright law.

[acpaquette]

Overall looks great! A couple smaller things to address then this should be good

[acpaquette]

Similar to the last comment, all changes look good. But bandtrim has been converted to a callable app. We might want to pick another example. An option would be to point to bandtrim before it was converted. See here:
https://github.com/DOI-USGS/ISIS3/tree/a28654ac754852f188be4c9710d001172ba459ac/isis/src/base/apps/bandtrim

[acpaquette]

I would avoid running ninja/make install, without an install directory this will place the binaries into the conda environment. This applies to all of these sections that mention building with either make or ninja

Generally, it's better to just run ninja and then add $ISISROOT/bin/ to your $PATH

The full command would look something like export PATH=$ISISROOT/bin:$PATH