Documentation for the web playground

Also mentioned it's out-of-development status; resolves #875

docs/book/src/SUMMARY.md

@@ -26,11 +26,7 @@
 
 # The Topiary Playground
 
-<!----------------------------------------------------------------------
-TODO: Documentation for the web-based playground; see issue #875
------------------------------------------------------------------------>
-
-- [TODO]()
+- [Web playground](playground/web.md)
 - [TTY playground](playground/tty.md)
 
 # Guides

docs/book/src/playground/tty.md

@@ -17,8 +17,10 @@ The INPUT_SOURCE is optional. If not specified, it defaults to trying
 to find the bundled integration test input file for the given language.
 ```
 
-For example, the playground can be run in a terminal emulator pane, with
-your editor of choice open in another.
+The script itself is very basic -- that cannot be overstated! -- but it
+provides a fast enough feedback loop to be useful. For example, the
+playground can be run in a terminal emulator/multiplexer pane, with your
+editor of choice open in another.
 
 <div class="warning">
 The use of inotify limits this tool to Linux systems, only.

docs/book/src/playground/web.md

@@ -0,0 +1,38 @@
+# Web playground
+
+<div class="warning">
+
+As of October 2024, the WASM-based web playground is _not_ under active
+development and has diverged from newer releases of Topiary. The web
+playground is kept online, for demonstrative purposes, but newer
+features have **not been implemented**.
+
+Please see GitHub to [assess the divergence](https://github.com/tweag/topiary/compare/playground...main).
+
+</div>
+
+The [Topiary web playground](https://topiary.tweag.io/playground) is a
+browser-based tool for experimenting with Topiary. It provides three
+editor panes:
+
+1. **Query:** This editor pane lets you define the Topiary formatting
+   queries to format the given language.
+
+2. **Input:** This editor pane allows you to set the input code in the
+   given language.
+
+3. **Output:** This editor pane (which is read only) shows the result of
+   Topiary formatting (i.e., the defined queries run against the input).
+
+The given language is set by a drop-down at the top of the window.
+Changing this will set the appropriate Tree-sitter grammar and populate
+the "Query" and "Input" panes with the default formatting queries for
+that language, as shipped with Topiary, and the sample testing input for
+that language; the "Output" pane will then be generated appropriately.
+
+By default, the web playground will format the input on-the-fly (i.e.,
+on changes to either the "Query" or "Input" panes). This option can be
+disabled with the appropriate checkbox and the "Format" button can be
+used as an alternative. Otherwise, options are provides that mimic the
+`--skip-idempotence` and `--tolerate-parsing-errors` flags to the
+Topiary CLI (see [`topiary format`](../cli/usage/format.md)).