feat(route): Start on support for generic discourse forums. (#13063)

* Start on support for generic discourse forums.

* Add doc.

* Update doc.

* Add exception.

* Update doc.

* Update bbs.md

* test: add discourse config

* docs: fix typo

* fix: guard condition

---------

lib/v2/discourse/posts.js

@@ -0,0 +1,28 @@
+const config = require('@/config').value;
+const got = require('@/utils/got');
+const RSSParser = require('@/utils/rss-parser');
+
+module.exports = async (ctx) => {
+    if (!config.discourse.config[ctx.params.configId]) {
+        throw Error('Discourse RSS is disabled due to the lack of <a href="https://docs.rsshub.app/install">relevant config</a>');
+    }
+    const { link, key } = config.discourse.config[ctx.params.configId];
+
+    const feed = await RSSParser.parseString(
+        (
+            await got(`${link}/posts.rss`, {
+                headers: {
+                    'User-Api-Key': key,
+                },
+            })
+        ).data
+    );
+
+    feed.items = feed.items.map((e) => ({
+        description: e.content,
+        author: e.creator,
+        ...e,
+    }));
+
+    ctx.state.data = { item: feed.items, ...feed };
+};