Add tutorials for blend animation and retargeting

[TokageItLab]

Added a section about the Blend animation which has been changed significantly in Godot 4.0, and a new page describing about the importer retarget.

In particular, the scene importer does not have a class reference, so it is necessary to explain it in the documentation.

Fixes godotengine/godot#59536
Fixes godotengine/godot#65771

[mhilbrunner]

Left a few comments. All in all, I feel like this PR needs some attention by a natural speaker, as I kind of struggled with whether my difficulty in following along was because of the language or due to my lack of familiarity with the topic.
I've left notes where I could to indicate where I think improvements could be made, but if someone like @clayjohn or someone else could give this a glance too, I'd be thankful. :)

Besides that, we need to come to a decision regarding the use of .webp for images and animated images, I'll try to get that done this week before we merge this.

Thanks for contributing!

[mhilbrunner]

We just decided on the move to .webp, and seeing the file size savings here, this indeed seems to be a good improvement :)

If you can replace the .pngs with .webp (lossless if <250kb, otherwise use quality settings that seem like the best fit) and squash commits, that'd be appreciated!

[TokageItLab]

@mhilbrunner

quality settings

Is there clear criteria like a target size?

[TokageItLab]

@mhilbrunner I changed the images to webp and squashed commits. PNG images have been made webP with lossless.

There is no clear target size given for the animated images, but I compressed them with quality=75 as ffmpeg default for now. It appears a bit noisy, but I think it's acceptable. If necessary, I will re-encode with cleaner settings. FYI if use quality=100, these animated images size will be about tripled.

[mhilbrunner]

Nice work! This is currently in the Asset Pipeline section, which makes sense as it mostly relates to import settings and importing skeletons I guess (from our/the engine's perspective).

Question is if a new user would be more likely to look under the Animation section? Opinions? I'm fine with leaving it where it is, just want to bring it up.

[clayjohn]

Looks mostly good! I left some suggestions where the language can be cleaned up a bit.

One thing that I did not comment on in the doc is the use of capitals for important terms. I found that I wasn't sure if things were capitalized because they matched names in the animation system (i.e. they are properties or class names) or because they were something the document was referring to multiple times.

In general, I think it is more clear to readers to:

1. Link to classes when speaking about a class (like SkeletonProfileHumanoid)
2. use backticks to put things in a code block when you are referring to a Godot property or method

[TokageItLab]

@mhilbrunner @clayjohn I embedded links for notable classes and used code brackets for properties in the importer.

About some camel words:

• Skeleton: This refers to the Skeleton3D class, but it is always "3D" added, which is annoying and I shortened
• Position track: This is used to refer to the animation track, which is clearly a separate type, but it is defined by enum not independent class
• Bone Rest, Bone Name: These exist as rest and name properties, but there are no properties called bone_rest or bone_name

[mhilbrunner]

At this point, I'm happy to merge as-is and iterate where needed. Thanks for these huge additions!