zulip_bots: Rename bots/{bot}/readme.md -> bots/{bot}/doc.md.

doc.md better describes the style of documentation that will live
inside these files, since we want these to be similar to our
webhooks' doc.md files in terms of how these are rendered and
composed of Markdown macros.

tools/custom_check.py

@@ -222,7 +222,7 @@ def build_custom_checkers(by_lang):
                 failed = True
 
         markdown_docs_length_exclude = {
-            "zulip_bots/zulip_bots/bots/converter/readme.md",
+            "zulip_bots/zulip_bots/bots/converter/doc.md",
             "tools/server_lib/README.md",
         }
         for fn in by_lang['md']: