Document image sizes management

[fritzmg]

Currently the Theme Manager documentation still lacks an article about the Image Sizes Manager. Plus it should show how to define pre-defined image sizes via the config.yml and also show how to translate the labels for pre-defined image sizes for the back end (see contao/contao#2589).

@ausi do you want to work on this by any chance? 😁 Since it's a daunting task describing all the settings and possibilities, may be you could simply copy RockSolid's blog post? I can then provide the English translation + the documentation for the config.yml and image_sizes.*.yml translation. Imho this would suffice initially. @netzarbeiter what do you think?

The developer documentation could then also link to the new article for more information about defining image sizes.

[m-vo]

We should also incorporate the changes from contao/contao#2243 somewhere in the section about image sizes from config.yml.

[fritzmg]

Ok, so I actually went ahead and documented the contao.image.sizes configuration in the dev docs and how to translate them for the back end (9213b29). My suggestion would be to document the back end image size settings in the manual, plus an outline on how to configure image sizes via contao.image.sizes (and how to translate them for the back end) with a link to the dev docs for a more detailed description.

We should also incorporate the changes from contao/contao#2243 somewhere in the section about image sizes from config.yml.

Also done in e5a3994 now :)