docs: fix broken documentation links

Update outdated docs.datajoint.com URLs to new paths:
- diagram.py: /how-to/installation/
- heading.py: /how-to/migrate-from-0x/
- expression.py: /how-to/migrate-from-0x/

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: dimitri-yatsenko

src/datajoint/diagram.py

@@ -48,7 +48,7 @@ class Diagram:
 
         See Also
         --------
-        https://docs.datajoint.com/core/datajoint-python/0.14/client/install/
+        https://docs.datajoint.com/how-to/installation/
         """
 
         def __init__(self, *args, **kwargs) -> None:

src/datajoint/expression.py

@@ -596,7 +596,7 @@ def fetch(self):
 
         For single-row fetch, use fetch1() which is unchanged.
 
-        See migration guide: https://docs.datajoint.com/migration/fetch-api
+        See migration guide: https://docs.datajoint.com/how-to/migrate-from-0x/
         """
         raise AttributeError(
             "fetch() has been removed in DataJoint 2.0. "

src/datajoint/heading.py

@@ -466,8 +466,8 @@ def _init_from_database(self) -> None:
                 except StopIteration:
                     if original_type.startswith("external"):
                         raise DataJointError(
-                            f"Legacy datatype `{original_type}`. Migrate your external stores to datajoint 0.12: "
-                            "https://docs.datajoint.io/python/admin/5-blob-config.html#migration-between-datajoint-v0-11-and-v0-12"
+                            f"Legacy datatype `{original_type}`. See migration guide: "
+                            "https://docs.datajoint.com/how-to/migrate-from-0x/"
                         )
                     # Not a special type - that's fine, could be native passthrough
                     category = None