Update README and docs index for v2

Update examples to use v2 command syntax with -i flag and alias
defaults. Add quick start section showing alias add/default/default-db
workflow. Add introspection commands section. Add environment.md to
docs toctree.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/index.md

@@ -21,15 +21,10 @@ If you also have Datasette installed in the same environment it will register it
 ```bash
 datasette install dclient
 ```
-This means you can run any of these commands using `datasette dt` instead, like this:
+This means you can run any of these commands using `datasette dc` instead, like this:
 ```bash
 datasette dc --help
-datasette dc query https://latest.datasette.io/fixtures "select * from facetable limit 1"
-```
-You can install it into Datasette this way using:
-
-```bash
-datasette install dclient
+datasette dc query fixtures "select * from facetable limit 1" -i latest
 ```
 
 ## Contents
@@ -42,4 +37,5 @@ queries
 aliases
 authentication
 inserting
+environment
 ```