Add docs for importing pilot models

[GeckoEidechse]

Harb. on the Discord server already wrote the documentation, it simply needs to be ported to ModdingDocs.

Link to Google Doc source can be found here: https://docs.google.com/document/d/1KeFiHYNT9fGK08X5o4LNGDRV9iMYuSap6FQqQgV9so4/edit

When submitting the PR, make sure to credit Harb. as co-author via Co-authored-by: Harb. in the commit message (unless it's Harb that's making the PR :P)

[IJARika]

fails to mention you need a source game installed. valid options are, in order of preference: portal 2, alien swarm, source filmmaker. also fails to mention you need to select one of these as the game when compiling with crowbar.