Update documentation and handling to use a crates/collab/seed.json (#10874)

Updates `collab` to accept a `seed.json` file that allows you to
override the defaults. Updated the `README` in collab to just have
directions inside instead of redirecting the developer to the website.

Release Notes:

- N/A

Co-authored-by: Max <max@zed.dev>

.gitignore

@@ -6,7 +6,7 @@
 /plugins/bin
 /script/node_modules
 /crates/theme/schemas/theme.json
-/crates/collab/.admins.json
+/crates/collab/seed.json
 /assets/*licenses.md
 **/venv
 .build