docs: Move migration-renumbering.md to subsystems/.

It belongs in that part of the documentation.  We also update the
links in docs/subsystems/schema-migrations.md.

docs/subsystems/index.rst

@@ -16,6 +16,7 @@ Subsystems Documentation
    management-commands
    front-end-build-process
    schema-migrations
+   migration-renumbering
    html_css
    hashchange-system
    emoji

docs/subsystems/schema-migrations.md

@@ -30,7 +30,7 @@ migrations.
   fix this, you can either run `./tools/renumber-migrations` which
   renumbers your migration(s) and fixes up the "dependencies" entries in your
   migration(s), and then rewrite your git history as needed, or you can do it
-  manually. There is a tutorial [here](../migration-renumbering.html) that
+  manually. There is a tutorial [here](migration-renumbering.html) that
   walks you though that process.
 
 * **Atomicity**.  By default, each Django migration is run atomically

tools/linter_lib/custom_check.py

@@ -666,7 +666,7 @@ def build_custom_checkers(by_lang):
             "docs/development/setup-vagrant.md",
             # Have wide output in code blocks
             "docs/subsystems/logging.md",
-            "docs/migration-renumbering.md",
+            "docs/subsystems/migration-renumbering.md",
             # Have curl commands with JSON that would be messy to wrap
             "zerver/webhooks/helloworld/doc.md",
             "zerver/webhooks/trello/doc.md",