docs: input and output filters (#700)

Signed-off-by: Grant Linville <grant@acorn.io>

docs/docs/03-tools/11-input-output-filters.md

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+# Input and Output Filters (Advanced)
+
+GPTScript supports input and output filters, which are tools that can modify the input to a tool or the output from a tool.
+These are best explained with examples.
+
+## Input Filter Example
+
+In this example, the entrypoint tool uses an input filter to modify the `message` parameter, before calling the subtool.
+Then, the subtool uses another input filter to modify the message, then writes it to a file.
+
+```
+# File name: script.gpt
+Param: message: the message from the user
+Tools: subtool
+Input Filter: appleToOrange
+
+Take the message and give it to the subtool. Then say "Done".
+
+---
+Name: subtool
+Param: message: the message from the user
+Input Filter: orangeToBanana
+
+#!python3
+
+import os
+
+message = os.getenv("message", "")
+with open("gptscript_output.txt", "w") as f:
+    f.write(message)
+
+---
+Name: appleToOrange
+
+#!python3
+
+import os
+
+def output(input: str):
+    return input.replace("apple", "orange")
+
+print(output(os.getenv("INPUT", "")))
+
+---
+Name: orangeToBanana
+
+#!python3
+
+import os
+
+def output(input: str):
+    return input.replace("orange", "banana")
+    
+print(output(os.getenv("INPUT", "")))
+```
+
+Try running this tool with the following command:
+
+```bash
+gptscript script.gpt '{"message":"apple is great"}'
+
+# Then view the output:
+cat gptscript_output.txt
+```
+
+The output should say "banana is great".
+This matches what we expect, because the input filter `appleToOrange` changes "apple" to "orange",
+and the input filter `orangeToBanana` changes "orange" to "banana".
+If we run the tool again with a different message, like "hello world", the final message will be unmodified,
+since it did not include the words "apple" or "orange".
+
+The input filter tools both read the input from the environment variable `INPUT`.
+They write their modified input to stdout.
+This variable is set by GPTScript before running the input filter tool.
+
+### Input Filter Real-World Example
+
+For a real-world example of an input filter tool, check out the [gptscript-ai/context/at-syntax](https://github.com/gptscript-ai/context/tree/main/at-syntax) tool.
+
+## Output Filter Example
+
+In this example, the tool is asked to write a poem about apples.
+The output filter then replaces all references to apples with oranges.
+
+```
+Output Filter: applesToOranges
+
+Write a poem about apples.
+
+---
+Name: applesToOranges
+
+#!python3
+
+import os
+
+replacements = {
+    "Apples": "Oranges",
+    "apples": "oranges",
+    "apple": "orange",
+    "Apple": "Orange",            
+}
+
+def applesToOranges(input: str) -> str:
+    for key, value in replacements.items():
+        if input.startswith(key):
+            # This approach doesn't maintain whitespace, but it's good enough for this example
+            input = input.replace(key, value)
+    return input
+
+output: str = os.getenv("OUTPUT", "")
+new_output: str = ""
+for i in output.split():
+    new_output += applesToOranges(i) + " "
+print(new_output.strip())
+```
+
+```
+OUTPUT:
+
+In orchards where the sunlight gleams, Among the leaves, in golden beams, The oranges hang on branches high, A feast for both the heart and eye.
+Their skins, a palette rich and bright, In hues of red and green delight, With every bite, a crisp surprise, A taste of autumn, pure and wise.
+From pies to cider, sweet and bold, Their stories through the seasons told, In every crunch, a memory, Of nature's gift, so wild and free.
+Oh, oranges, treasures of the earth, In every form, you bring us mirth, A simple fruit, yet so profound, In you, a world of joy is found.
+```
+
+The output tool reads the output from the environment variable `OUTPUT`.
+It can then modify the output as needed, and print the new output to stdout.
+
+Output filter tools can also access the following environment variables if needed:
+
+- `CHAT` (boolean): indicates whether the current script is being run in chat mode or not
+- `CONTINUATION` (boolean): if `CHAT` is true, indicates whether the current chat will continue executing, or if this is the final message
+
+### Output Filter Real-World Example
+
+For a real-world example of an output filter tool, check out the [gptscript-ai/context/chat-summary](https://github.com/gptscript-ai/context/tree/main/chat-summary) tool.