Add basic docs for the lang module

Signed-off-by: Gavin Howard <gavin@yzena.com>

docs/lang/api.md

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+# API
+
+```{eval-rst}
+.. doxygengroup:: lang
+```

docs/lang/design.md

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+# Design
+
+## Requirements
+
+* Returning an error code from everything should not be necessary.
+* The lexer should be able to switch modes.

docs/lang/main.md

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+# Language Lexing and Parsing (`lang`)
+
+This module includes common plumbing for lexing and parsing languages.
+
+This includes:
+
+* Lex modes.
+* Lex variables.
+* Error handling.
+* Basic parsing.
+
+:::{toctree}
+---
+maxdepth: 2
+---
+api.md
+design.md
+:::