Rewrite Commands section's landing page

- reST -> MyST
- Roughly group the commands, based on what they work on.

docs/html/cli/index.md

@@ -0,0 +1,48 @@
+# Commands
+
+The general options that apply to all the commands listed below can be
+found [under the `pip` page in this section](pip).
+
+```{toctree}
+:maxdepth: 1
+:hidden:
+
+pip
+```
+
+```{toctree}
+:maxdepth: 1
+:caption: Environment Management and Introspection
+
+pip_install
+pip_uninstall
+pip_list
+pip_freeze
+pip_check
+```
+
+```{toctree}
+:maxdepth: 1
+:caption: Handling Distribution Files
+
+pip_download
+pip_wheel
+pip_hash
+```
+
+```{toctree}
+:maxdepth: 1
+:caption: Package Index information
+
+pip_show
+pip_search
+```
+
+```{toctree}
+:maxdepth: 1
+:caption: Managing pip itself
+
+pip_cache
+pip_config
+pip_debug
+```