MKDocs conversion (#278)

Closes #252, Closes #250

## Changes
- ReplayWeb.page uses MKDocs for documentation now!
- Requires both
[`mkdocs-macros`](https://mkdocs-macros-plugin.readthedocs.io/en/latest/)
and [`mkdocs-redirects`]
(https://github.com/mkdocs/mkdocs-redirects) plugins to be installed
- Redirects existing docs pages to their new locations under either
`/embedding` or `/user-guide`
- Unused images have been removed
- Adds new ReplayWeb.page branding!

All pages have received copy edits. All references to future (or
previously) planned features have been removed. More major content
changes of note are as follows:

### Supported Locations
- Made into a page, no longer a table. Locations are either supported or
not listed.

### Offline Use 
- Removes browser cache section, adds improved PWA install and uninstall
instructions.
- Removes navigation instructions to GitHub's releases section in favour
of a link

### Supported Formats
- Moved to the User Guide homepage
- Updated the format support

### Exploring Archives
- Example links for showcasing features have been moved to their
respective feature documentation section on this page

### Remove Usage, Examples, and Troubleshooting Pages
- Usage content is covered in the user guide index
- Troubleshooting content has been added to a question section on the
getting started page about WARC vs WACZ.
- Examples are now Featured Projects on the docs homepage!

### Build Process
- Removes Jekyll & related site assets.
- Adds new GitHub action for publishing
- Adds a python macro to dynamically load the replayweb.page version from package.json
- Relocates replayweb.page site content under /site (including docs)
- Some assets (favicon, logo) and sw.js/ui.js automatically copied into /site

---------
Co-authored-by: Ilya Kreymer <ikreymer@gmail.com>

package.json

@@ -73,17 +73,17 @@
     "index.html"
   ],
   "scripts": {
-    "build": "webpack --mode production",
-    "build-dev": "webpack --mode development",
-    "build-docs": "bundle install; bundle exec jekyll build",
+    "build": "webpack --mode production && cp dist/ui.js site/ui.js",
+    "build-dev": "webpack --mode development && cp dist/ui.js site/ui.js",
+    "build-docs": "yarn run build && cd mkdocs && mkdocs build",
+    "start-prod": "cd site && http-server -p 9990 --cors",
     "start-dev": "webpack serve --mode development",
-    "start-prod": "http-server -p 9990 --cors",
+    "start-docs": "yarn run build-dev && cd mkdocs && mkdocs serve",
+    "start-electron": "NODE_ENV=development electron ./dist/electron.js $1",
     "test-start-embed": "cd tests/embed; http-server -p 8020",
     "test-start-sandbox": "cd tests/embed/sandbox; http-server -p 8030",
     "pack": "CSC_IDENTITY_AUTO_DISCOVERY=false electron-builder --publish never",
     "pack-signed": "electron-builder",
-    "start-electron": "NODE_ENV=development electron ./dist/electron.js $1",
-    "start-jekyll": "yarn run build-docs && bundle exec jekyll serve",
     "dist": "yarn run build && yarn run pack",
     "dist-dev": "yarn run build-dev && yarn run pack",
     "release": "yarn run build && electron-builder",