doc: Add guides and GitHub workflow for doc sync

This PR:
- adds all the guides (in markdown format) that is published at https://docs.corelightning.org/docs
- adds a github workflow to sync any future changes made to files inside the guides folder
- does not include API reference (json-rpc commands). Those will be handled in a separate PR since they're used as manpages and will require a different github workflow

Note that the guides do not exactly map to their related files in doc/, since we reorganized the overall documentation structure on readme for better readability and developer experience. For example, doc/FUZZING.md and doc/HACKING.md#Testing are merged into testing.md in the new docs. As on the creation date of this PR, content from each of the legacy documents has been synced with the new docs. Until this PR gets merged, I will continue to push any updates made to the legacy documents into the new docs.

If this looks reasonable, I will add a separate PR to clean up the legacy documents from doc/ (or mark them deprecated) to avoid redundant upkeep and maintenance.

Changelog-None

.github/workflows/rdme-docs-sync.yml

@@ -0,0 +1,23 @@
+# This GitHub Actions workflow was auto-generated by the `rdme` cli on 2023-04-22T13:16:28.430Z
+# You can view our full documentation here: https://docs.readme.com/docs/rdme
+name: ReadMe GitHub Action 🦉
+
+on:
+  push:
+    branches:
+      # This workflow will run every time you push code to the following branch: `master`
+      # Check out GitHub's docs for more info on configuring this:
+      # https://docs.github.com/actions/using-workflows/events-that-trigger-workflows
+      - master
+
+jobs:
+  rdme-docs:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out repo 📚
+        uses: actions/checkout@v3
+
+      - name: Run `docs` command 🚀
+        uses: readmeio/rdme@v8
+        with:
+          rdme: docs guides/ --key=${{ secrets.README_API_KEY }} --version=23.02