Plasma missing from TOC

[isaacgsmith]

Problem description

After renaming a file in PR #1371, docs/index.rst was not updated, and therefore docs/physics/plasma.rst does not show up in the main TOC.
[this should explain why the current behaviour is a problem and why the expected output is a better solution.]

Expected Output

Plasma should appear under "The Physics of Tardis" in the documentation sidebar.

This will be fixed ASAP.