docs: create a basic homepage for the docs website

Before, https://martinvonz.github.io/jj/latest redirected to
https://martinvonz.github.io/jj/latest/install-and-setup.html.

Now, it will direct people to a basic page with a link to
the repo and a few other useful pages.

An additional motivation is the desire to have a homepage to
link to from
#2273 (comment).

Better something very basic than nothing.

docs/index.md

@@ -0,0 +1,21 @@
+# Jujutsu—a version control system
+
+## Welcome to `jj`'s documentation website!
+
+The complete list of the documentation pages available should be visible in the
+sidebar.
+
+Additional help is available using the `jj help` command if you have `jj`
+installed.
+
+You may want to jump to:
+
+- Documentation for the [latest released version of `jj`](https://martinvonz.github.io/jj/latest).
+- Documentation for the [unreleased version of `jj`](https://martinvonz.github.io/jj/prerelease) that corresponds to the `main` branch of the `jj` repo.
+
+## Some useful links
+
+- [GitHub repo for `jj`](https://github.com/martinvonz/jj)
+- [Installation instructions](install-and-setup.md)
+- [Tutorial](tutorial.md)
+- [GitHub workflows with `jj`](github.md)

mkdocs.yml

@@ -15,7 +15,6 @@ plugins:
     - search
     - redirects:
         redirect_maps:
-            'index.md': 'install-and-setup.md'
 
 # Not all of these may be necessary, especially since the material
 # theme substitutes for some of them
@@ -38,6 +37,8 @@ markdown_extensions:
 
 # This lists all the files that become part of the documentation
 nav:
+- 'Home': 'index.md'
+
 - 'Getting started':
       - 'Installation and Setup': 'install-and-setup.md'
       - 'Tutorial and Birds-Eye View': 'tutorial.md'