Show binary releases details on resolc-bin GH Pages site (#447)

## Description

For more easily finding the details of the resolc binary releases for
the various supported platforms, the plain JSON information will be
displayed on [resolc-bin](https://github.com/paritytech/resolc-bin)'s
[GitHub Pages website](https://paritytech.github.io/resolc-bin/).

Changes:
* Whenever there is a regular or nightly release:
* The corresponding workflow files will run a script to update
resolc-bin's `index.md` file (new file) with the updated JSON.
* `index.md` will be the file GitHub Pages serves as HTML (by default it
is built with Jekyll and the Markdown is processed by kramdown, hence
the syntax seen in the script).
* Documentation  updates

Thus, this will run on the next nightly release after this PR is merged.

## Resolved Issues

Closes #443

---------

Co-authored-by: xermicus <cyrill@parity.io>

.github/workflows/book.yml

@@ -37,6 +37,6 @@ jobs:
       - name: Compare with committed docs
         run: |
           if ! diff -r docs-tmp docs 2>/dev/null; then
-            echo "docs/ is not up-to-date. Run make test-book and commit the docs/ directory"
+            echo "docs/ is not up-to-date. Run \`make book\` and commit the docs/ directory"
             exit 1
           fi