Fix doc building

Currently building the html docs results in 1.8GB output, mostly due to some plots producing excessively large pdf files, the largest one being `tutorial/code_snippets/plotting_spectrograms.pdf` with 380 MB size. Also the corresponding extremely long build time seems responsible for problems with stalled automatic master docs building (plots that are included in the gallery as well are also duplicated regarding rendering time and output size).

I don't know why this has become an issue suddenly but the simplest solution is to just not render pdf images of the plots. I don't think they would be missed badly, the high-resolution png images should be enough for demonstration purposes. When leaving out pdf images, the images in the html docs build only take up around 20 MB overall, build time is reduced to roughly 20% (under 10 mins now on my machine, with less RAM this might have an even higher impact).

The other option would be to modify all those plots that blow up the docs with their pdf rendered version. However this could again run into problems in the future when new big plots get added. The largest plots in the tutorial/gallery currently are as follows:
```
  376.4MiB [##########]  plotting_spectrograms.pdf
   83.5MiB [##        ]  continuous_wavelet_transform_obspy.pdf
   30.4MiB [          ]  time_frequency_misfit_ex5_01.pdf
   30.3MiB [          ]  time_frequency_misfit_ex2_01.pdf
   30.3MiB [          ]  time_frequency_misfit_ex4_01.pdf
   30.3MiB [          ]  time_frequency_misfit_ex2_00.pdf
   30.3MiB [          ]  time_frequency_misfit_ex5_00.pdf
   30.2MiB [          ]  time_frequency_misfit_ex4_00.pdf
   30.2MiB [          ]  time_frequency_misfit_ex3_01.pdf
   30.2MiB [          ]  time_frequency_misfit_ex3_00.pdf
   15.3MiB [          ]  time_frequency_misfit_ex1.pdf
    9.9MiB [          ]  array_response_function.pdf
    5.3MiB [          ]  probabilistic_power_spectral_density.pdf
    4.7MiB [          ]  waveform_plotting_tutorial_3.pdf
    4.4MiB [          ]  beamforming_fk_analysis_2.pdf
    1.6MiB [          ]  waveform_plotting_tutorial_1.pdf
    1.4MiB [          ]  waveform_plotting_tutorial_4.pdf
```
Some inline/docstring image pdf are large as well:
```
   58.4MiB [##########]  c2b8f15de3.pdf
   30.3MiB [#####     ]  c8e4698e1a.pdf
   30.2MiB [#####     ]  66201a20b5.pdf
   30.2MiB [#####     ]  7b716b6695.pdf
    1.4MiB [          ]  dd9b1acca5.pdf
    1.3MiB [          ]  00ecd33332.pdf
    1.3MiB [          ]  70820feddf.pdf
    1.3MiB [          ]  6866cd20f7.pdf
    1.2MiB [          ]  c982bd5e86.pdf
    1.2MiB [          ]  490fae1285.pdf
    1.2MiB [          ]  f9ee82c45a.pdf
    1.2MiB [          ]  8e4acf8226.pdf
    1.2MiB [          ]  6aa0e91fa1.pdf
    1.2MiB [          ]  83018de5b4.pdf
```

As a side effect the pdf build of the tutorial had to be changed to use png versions of the plots (I set a slightly higher dpi for the default png images). It is much smaller now (~3-4 MB) with still acceptable images which is an advantage when printing.

I've also added the built tutorial pdf as a downloadable item in the html docs.