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>

README.md

@@ -48,8 +48,6 @@ ReplayWeb.page is built as a Node package can be installed using yarn:
 
 `yarn install`
 
-It is also a Jekyll site for use with GitHub pages. Jekyll is only needed to build the docs.
-
 The package provides various commands that can be used with yarn:
 
 - `yarn start-dev` - to run in dev mode with Webpack dev server on port 9990. Autobuilds dev assets.
@@ -62,10 +60,6 @@ The package provides various commands that can be used with yarn:
 
 - `yarn dist` - to build production assets + Electron app (in dist/)
 
-- `yarn build-docs` - to build the Jekyll docs (for local use) and run Jekyll. Requires Ruby.
-
-- `yarn start-jekyll` - to build Jekyll docs and start Jekyll static server on port 4000.
-
 The static assets are placed in the root `index.html`, `sw.js` and `ui.js`, and can be used with any HTTP server.
 
 For service workers to work, they must be served from either localhost or an HTTPS endpoint.