docs(developer-guide): add a Developer Guide section

Two pages, both aimed at SAS developers rather than end users:

- Filter in a Pre-Edit Hook: what editors/getdata returns (sasparams.FILTER_TEXT
  and the query clauses, never FILTER_RK), when PRE_EDIT_HOOK runs relative to
  the filter, and a worked example that widens a date filter to the previous
  quarter - re-reading the table so the rows match, and keeping Row Level
  Security applied to both branches.
- One Table, Two Rule Sets: moved from the datacontroller.io blog, so the
  developer-facing material lives in one place. Images moved to docs/img/.

mkdocs.yml

@@ -14,6 +14,9 @@ nav:
     - Table Viewer: dcu-tableviewer.md
     - ViewBoxes: viewboxes.md
     - Admin Services: admin-services.md
+- Developer Guide:
+    - Filter in a Pre-Edit Hook: developer-guide/filter-pre-edit-hook.md
+    - One Table, Two Rule Sets: developer-guide/two-rule-sets-one-table.md
 - Table Guide:
     - MPE_AUDIT: tables/mpe_audit.md
     - MPE_COLUMN_LEVEL_SECURITY: tables/mpe_column_level_security.md