BD documentation includes pdf graphics which don't typically render

[andrew-platt]

The BeamDyn documentation includes a few pdf versions of figures, which don't render on readthedocs. These should be converted to png format sometime.

Figures 4.33 and 4.44 here: https://openfast.readthedocs.io/en/main/source/user/beamdyn/theory.html (https://openfast.readthedocs.io/en/main/source/user/beamdyn/theory.html#id15)

Figure 4.35 here: https://openfast.readthedocs.io/en/main/source/user/beamdyn/appendix.html#id1