docs: Add named workspaces notes for conda users

[soapy1]

Description

Follow up to #5227 (ref).

Adds a comparison between conda's and pixi's named workspaces in the switching from conda/mamba docs.

How Has This Been Tested?

See changes locally:

$ pixi run docs 

Checklist:

• I have made corresponding changes to the documentation

[baszalmstra]

Nice!

Are there still caviats that should be considered with this approach that we should maybe record here?

[soapy1]

Heya @baszalmstra, thanks for taking a look! I've added two tips to:

• describe the difference between how a user can create a named environment/workspace in conda/pixi
• describe pruning workspace from the registry that have become disassociated

I think these cover the main questions and caviats that users may have about this approach.