api-docs: Move markdown files to top level directory.

- Updates `.prettierignore` for the new directory.
- Updates any reference to the API documentation directory for
  markdown files to be `api_docs/` instead of `zerver/api/`.
- Removes a reference link from `docs/documentation/api.md` that
  hasn't referenced anything in the text since commit 0542c60.
- Update rendering of API documentation for new directory.

api_docs/send-message.md

@@ -0,0 +1,77 @@
+{generate_api_header(/messages:post)}
+
+## Usage examples
+
+{start_tabs}
+
+{generate_code_example(python)|/messages:post|example}
+
+{generate_code_example(javascript)|/messages:post|example}
+
+{tab|curl}
+
+``` curl
+# For stream messages
+curl -X POST {{ api_url }}/v1/messages \
+    -u BOT_EMAIL_ADDRESS:BOT_API_KEY \
+    --data-urlencode type=stream \
+    --data-urlencode 'to="Denmark"' \
+    --data-urlencode topic=Castle \
+    --data-urlencode 'content=I come not, friends, to steal away your hearts.'
+
+# For private messages
+curl -X POST {{ api_url }}/v1/messages \
+    -u BOT_EMAIL_ADDRESS:BOT_API_KEY \
+    --data-urlencode type=private \
+    --data-urlencode 'to=[9]' \
+    --data-urlencode 'content=With mirth and laughter let old wrinkles come.'
+```
+
+{tab|zulip-send}
+
+You can use `zulip-send`
+(available after you `pip install zulip`) to easily send Zulips from
+the command-line, providing the message content via STDIN.
+
+```bash
+# For stream messages
+zulip-send --stream Denmark --subject Castle \
+    --user othello-bot@example.com --api-key a0b1c2d3e4f5a6b7c8d9e0f1a2b3c4d5
+
+# For private messages
+zulip-send hamlet@example.com \
+    --user othello-bot@example.com --api-key a0b1c2d3e4f5a6b7c8d9e0f1a2b3c4d5
+```
+
+#### Passing in the message on the command-line
+
+If you'd like, you can also provide the message on the command-line with the
+`-m` or `--message` flag, as follows:
+
+
+```bash
+zulip-send --stream Denmark --subject Castle \
+    --message 'I come not, friends, to steal away your hearts.' \
+    --user othello-bot@example.com --api-key a0b1c2d3e4f5a6b7c8d9e0f1a2b3c4d5
+```
+
+You can omit the `user` and `api-key` parameters if you have a `~/.zuliprc`
+file.
+
+{end_tabs}
+
+## Parameters
+
+{generate_api_arguments_table|zulip.yaml|/messages:post}
+
+{generate_parameter_description(/messages:post)}
+
+## Response
+
+{generate_return_values_table|zulip.yaml|/messages:post}
+
+{generate_response_description(/messages:post)}
+
+#### Example response(s)
+
+{generate_code_example|/messages:post|fixture}