Give the brief and rules axis categories their own translation keys

The build got past MDX and died in the RU locale: putting the brief and
rules articles on the platform's axes made `Domain logic`, `View logic` and
`Physical model` appear three times each in the `learn` sidebar, and
Docusaurus derives a translation key from `category.key ?? category.label`,
so three categories claimed one key.

`key` is in the 3.9.2 sidebar schema (`Joi.string()` on the category item),
so the six categories under Brief and Rules now carry `brief-*` / `rules-*`
keys and the paradigm ones keep theirs. Checked by running the library's
own rule over the sidebar: 82 categories, no duplicate key — and the same
check on the pushed version reproduces the three collisions.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012YrYJtJ6hR2qJsk7FxjTjW

Author: 2 people

docs/sidebars.js

@@ -661,6 +661,7 @@ module.exports = {
           items: [
             {
               type: 'category',
+              key: 'brief-domain-logic',
               label: 'Domain logic',
               items: [
                 'brief/Brief_properties',
@@ -674,6 +675,7 @@ module.exports = {
             },
             {
               type: 'category',
+              key: 'brief-view-logic',
               label: 'View logic',
               items: [
                 'brief/Brief_forms',
@@ -684,6 +686,7 @@ module.exports = {
             },
             {
               type: 'category',
+              key: 'brief-physical-model',
               label: 'Physical model',
               items: [
                 'brief/Brief_identification',
@@ -704,6 +707,7 @@ module.exports = {
           items: [
             {
               type: 'category',
+              key: 'rules-domain-logic',
               label: 'Domain logic',
               items: [
                 'rules/Rules_properties',
@@ -717,6 +721,7 @@ module.exports = {
             },
             {
               type: 'category',
+              key: 'rules-view-logic',
               label: 'View logic',
               items: [
                 'rules/Rules_forms',
@@ -727,6 +732,7 @@ module.exports = {
             },
             {
               type: 'category',
+              key: 'rules-physical-model',
               label: 'Physical model',
               items: [
                 'rules/Rules_modularity',