Make the POTCAR setup instructions clearer (#3630)

* Make the POTCAR setup instructions clearer

Closes #3629. I have always found the POTCAR setup instructions to be incredibly confusing, in part because some parts were never defined (e.g. `<MY_PSP>`) and in other parts because things are quite out-of-sync with the POTCAR structure offered by VASP as of the last several years. This is an attempt to make it clearer.

Signed-off-by: Andrew S. Rosen <asrosen93@gmail.com>

* Typo fix

Signed-off-by: Andrew S. Rosen <asrosen93@gmail.com>

* Further clarification

Signed-off-by: Andrew S. Rosen <asrosen93@gmail.com>

* Further clarification

Signed-off-by: Andrew S. Rosen <asrosen93@gmail.com>

* Update installation.md

Signed-off-by: Andrew S. Rosen <asrosen93@gmail.com>

* Update installation.md

Signed-off-by: Andrew S. Rosen <asrosen93@gmail.com>

* Update installation.md

Signed-off-by: Andrew S. Rosen <asrosen93@gmail.com>

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Signed-off-by: Andrew S. Rosen <asrosen93@gmail.com>