Add small tutorial on making existing mesh manifold with Blender

[tetrapod00]

Closes #10417.

Personally I do think this is a marginal inclusion, since it's documentation on using a specific addon for another program. But it's also a pretty small inclusion and addresses a potential pain point for users updating to 4.4.

I wrote the version of the tutorial that I think is the right scope for the official docs, and we can either approve it or not.

CC @fire

[skyace65]

While an argument could be made that this is out of scope I think this is small enough and justified enough to be included in the docs, I'm also ok with linking to a non-godot created extension since it's hosted on blender.org. We've done similar things to this in the past I still agree with like showing users how to make ico files because it's an obscure process.

[mhilbrunner]

I agree with Sky. Merged! @fire Feel free to ping here or on RC #docs if there are any concerns with this :) Noticed the request for your review too late.

[fire]

Looks good to me.

[fire]

Additional tips for people who want to make manifold in Blender coming from a search engine.

1. You can merge "near" vertices.
2. Another trick when there is a few vertices left is go into edit mode and select with traits of non-manifold and delete them.
3. The make manifold tutorial in this pull request can help here too.
4. Mix and match the listed techniques.