Add equation for length

We don't yet have an equation for calculating length, which might be useful:

```math
L = g \left( \frac{T}{2\pi} \right)^2

```

Locate the teaching-materials section where equations for scientific calculations are documented and review its surrounding formatting. Add the proposed length equation with the same conventions as nearby equations, then verify that the rendered material displays it correctly.

Written by the indexing model from the issue text.