Introduce RFC mdBook web page (#60)

- Closes https://github.com/polkadot-fellows/RFCs/issues/53
- Preview available [here](https://paritytech.github.io/RFCs/).

![Screenshot 2023-12-19 at 13 55
05](https://github.com/polkadot-fellows/RFCs/assets/12039224/ee06a7ad-231e-4077-a773-c6319661dfb6)


---

### What it does

- The workflow gathers the source markdown files of RFCs - approved
ones, and proposed ones (from open PRs).
- The proposed RFCs are separated into new (<7 days since PR created)
and stale (>30 days since PR updated).
- The RFCs are extended with a table of contents, and a link to the
source.
- The RFCs are build into a web page using
[mdBook](https://github.com/rust-lang/mdBook).
- The built web page gets pushed to `gh-pages` branch - to be published
in Github Pages.
- The workflow is triggered on merge to `main`, and periodically once a
day.

.github/scripts/build-mdbook-summary.js

@@ -0,0 +1,38 @@
+/**
+ * mdBook relies on the creation of a special SUMMARY.md file
+ * https://rust-lang.github.io/mdBook/format/summary.html
+ * This script constructs the summary out of the available source files.
+ */
+
+const fs = require('fs');
+const summaryPath = "mdbook/src/SUMMARY.md"
+
+module.exports = async ({github, context}) => {
+  fs.writeFileSync(summaryPath, "# Summary\n\n[Introduction](introduction.md)\n") // Starting point.
+
+  const appendRfcsToSummary = (dirPath) => {
+    for (const filename of fs.readdirSync(dirPath)) {
+      if (!filename.endsWith(".md")) continue;
+      const filePath = dirPath + filename
+      const text = fs.readFileSync(filePath)
+      const title = text.toString().split(/\n/)
+        .find(line => line.startsWith("# ") || line.startsWith(" # "))
+        .replace("# ", "")
+      // Relative path, without the src prefix (format required by mdbook)
+      const relativePath = filePath.replace("mdbook/src/", "")
+      fs.appendFileSync(summaryPath, `- [${title}](${relativePath})\n`)
+    }
+  }
+
+  fs.appendFileSync(summaryPath, "\n---\n\n# Approved\n\n")
+  appendRfcsToSummary("mdbook/src/approved/")
+
+  fs.appendFileSync(summaryPath, "\n---\n\n# Newly Proposed\n\n")
+  appendRfcsToSummary("mdbook/src/new/")
+
+  fs.appendFileSync(summaryPath, "\n---\n\n# Proposed\n\n")
+  appendRfcsToSummary("mdbook/src/proposed/")
+
+  fs.appendFileSync(summaryPath, "\n---\n\n# Stale\n\n")
+  appendRfcsToSummary("mdbook/src/stale/")
+}