Add tasknotes-spec as submodule and wire up docs nav

Add the specification repo as a git submodule at docs/spec and add
a Specification nav section in mkdocs.yml pointing at all spec files.

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "docs/spec"]
+	path = docs/spec
+	url = https://github.com/callumalpass/tasknotes-spec.git

docs/spec

@@ -0,0 +1 @@
+Subproject commit c61895f80ff9b315a558d45607c497a6ec2343ed

mkdocs.yml

@@ -91,6 +91,20 @@ nav:
     - Webhooks: webhooks.md
     - Workflows: workflows.md
     - Calendar Setup: calendar-setup.md
+  - Specification:
+    - Overview: spec/00-overview.md
+    - Terminology: spec/01-terminology.md
+    - Model & Mapping: spec/02-model-and-mapping.md
+    - Temporal Semantics: spec/03-temporal-semantics.md
+    - Recurrence: spec/04-recurrence.md
+    - Operations: spec/05-operations.md
+    - Validation: spec/06-validation.md
+    - Conformance: spec/07-conformance.md
+    - Compatibility & Migrations: spec/08-compatibility-and-migrations.md
+    - Configuration: spec/09-configuration.md
+    - Dependencies & Reminders: spec/10-dependencies-and-reminders.md
+    - Links: spec/11-links.md
+    - Changelog: spec/CHANGELOG.md
   - Migration Guide: migration-v3-to-v4.md
   - Development:
     - Translation Workflow: development/translations.md