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>

site/docs/user-guide/loading.md

@@ -0,0 +1,58 @@
+# Loading Archived Items
+
+ReplayWeb.page can load archived items from the local filesystem, or from remote locations. If you don't have any archived items yet, try archiving a website with [ArchiveWeb.page](https://archiveweb.page)!
+
+---
+
+## Loading Local Files
+
+From [replayweb.page](https://replayweb.page), press the _Choose File..._ button, pick any local archived item file from your computer, then press _Load_ to start viewing the file's archived content. 
+
+<img alt="screenshot of replayweb.page (online)" src="../../assets/replaywebpage-online-local.png">
+
+The archived item is loaded directly in the browser and is _not_ uploaded anywhere — your data never leaves your computer!
+
+Once loaded, the archived item will appear in the list of Loaded Archives and will be identified by its filename with a a `file://` URL as its source. This URL is not shareable as it doesn't exist outside of your computer.
+
+<figure>
+    <img alt="animated gif of replayweb.page opening an archived item" src="../../assets/replaywebpage-online.gif">
+    <figcaption>Loading an archived item into ReplayWeb.page and viewing a URL in the pages list</figcaption>
+</figure>
+
+To unload an archived item, click on the 'X'. Unloading an item does not delete the file from its original location.
+
+## Loading Files from a URL
+
+To load an archived item stored in a remote location, enter the URL of the file and press _Load_.
+
+The archived item will be downloaded, either fully or on-demand (if possible) and presented when ready.
+
+See the [Supported Locations](locations.md) page for details on supported external sources for serving archived items.
+
+## Loading Files from Google Drive
+
+In addition to loading remote content from URLs, ReplayWeb.page also supports loading WARC and WACZ files directly from Google Drive.
+
+### Add-on Installation
+
+First, install the [ReplayWeb.page Google Drive Addon](https://gsuite.google.com/u/2/marketplace/app/replaywebpage/160798412227).
+
+* Press the _Install_ button, Google will walk you through the steps of installation.
+* Once installed, Google will confirm the install and display where to find ReplayWeb.page in your drive.
+
+<figure>
+    <img alt="animated gif of replayweb.page google drive add-on install process" src="../../assets/replaywebpage-googledrive-install.gif">
+    <figcaption>Installing the ReplayWeb.page Google Drive add-on</figcaption>
+</figure>
+
+### Usage
+
+1. Navigate to any WARC or WACZ files that available on your Google Drive.
+
+2. Under the file's _More Actions_ :bootstrap-three-dots-vertical: menu, select _Open With_ and choose _ReplayWeb.page_ from the menu.
+
+This will redirect to ReplayWeb.page and begin loading the file. Items loaded from Google Drive will have a source URL starting with `googledrive://`.
+
+---
+
+Once an archived item has been successfully loaded, the app will display the archive viewer and you can begin exploring your loaded archive.