Developer docs for image sizing and cropping

[joemull]

Problem description

It is tricky to anticipate how images are key images treated in Janeway's media storage and how they are displayed for each of the themes, especially for large images.

Proposed solution - updated to reflect new scope of work

Refactor and make consistent the fallback logic for large images, and document it.

[joemull]

I started investigating this in a call with @S-Haime but decided it would be better to do as a part of the new developer docs.

Here is the first bit of that investigation:

• Press override image
  • Should be taller than wide, about 3 times as wide
  • The image is not cropped on upload. It is constrained by CSS in these ways:
    • OLH: to 60 px high or 180 px wide, whichever is exceeded first. The aspect ratio is maintained
    • Clean:
    • Material:

[joemull]

Consider doing this at the same time if it is quick:

• Pixelation of large images #4222

[S-Haime]

#2140
#3605