[docs] Add db migration tip for slow hardware instances. (#4457)

This PR adds a new section to the documentation to contain workarounds for running
GtS on slow hardware. Right now it only contains a procedure on how to run migrations
on a different database instance in case the original database is too slow to finish
a database migration in a timely manner.

Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4457
Co-authored-by: Daniël Franke <df@ponc.tech>
Co-committed-by: Daniël Franke <df@ponc.tech>

mkdocs.yml

@@ -38,7 +38,7 @@ plugins:
 extra:
   alternate:
     - name: English
-      link: /en/ 
+      link: /en/
       lang: en
     - name: 中文
       link: /zh-cn/
@@ -149,6 +149,7 @@ nav:
       - "admin/spam.md"
       - "admin/database_maintenance.md"
       - "admin/themes.md"
+      - "admin/slow_hardware.md"
   - "Federation":
       - "federation/index.md"
       - "federation/http_signatures.md"