ENH: doc of ridge functions

[sciunto]

Description

This PR aims at improving the doc of #3515 as these functions are designed for processing on vessels, I changed the image for a crop of retina. I added see also sections as well.

Cc @DavidBreuer

Checklist

• Docstrings for all functions
• Gallery example in ./doc/examples (new features only)
• Benchmark in ./benchmarks, if your changes aren't covered by an
  existing benchmark
• Unit tests
• Clean style in the spirit of PEP8

For reviewers

• Check that the PR title is short, concise, and will make sense 1 year
  later.
• Check that new functions are imported in corresponding __init__.py.
• Check that new features, API changes, and deprecations are mentioned in
  doc/release/release_dev.rst.
• Consider backporting the PR with @meeseeksdev backport to v0.14.x

[emmanuelle]

Thanks @sciunto ! The contrast of frangi is flattened because the higher values are at the border of the image, could you please increase the contrast (eg setting vmax) for this filter, since the result is quite good but it doesn't show here.

[emmanuelle]

We could also have more explanations about the differences between the different filters, but this can come in another PR because this PR is already an improvement.

[sciunto]

Do you think it is better to set a vmax for a particular case or crop all images of 4 pixels on each edge and add a note saying that Frangi leads to an edge effect removed in this example?

PS: I'm discovering these filters and I'm not familiar at all with these algorithms.

[jni]

I'm confused — doesn't matplotlib automatically stretch the contrast for each image?

[jni]

Oh, sorry, misread @emmanuelle's comment. Hmmm. I'm not sure what the best answer is here. @sciunto go with whatever you think is prettiest I reckon. =)

[emmanuelle]

@sciunto I would stretch the contrast for this particular case and explain in the text that Frangi filter does this special thing at the border (which will be visible because these pixels will be saturated). But I also leave it to your personal taste, the important thing is that this subplot is not pitch black and users think frangi is worthless.

[sciunto]

I forgot about this PR. I put a crop to improve the rendering.

[jni]

Looks great, thanks @sciunto!!!