api docs: Replace most markdown files with a common template.

This PR adds a basic .md template that is followed by lot of /api
pages. Since we have recently done the migration work to ensure that
our REST API documentation pages for individual endpoints are almost
all identical files following a common pattern, we can now get the
payoff of deleting them all in favor of a shared template.

This removes 2000 lines of somewhat finicky configuration from the
codebase, and thus should save significant effort when documenting new
API endpoints in the future.

The markdown files for endpoints or other pages which deviate from the
standard template remain, and the docs are instead generated from
those files using the existing system.

zerver/lib/templates.py

@@ -150,7 +150,6 @@ def render_markdown_path(
     md_engine.reset()
 
     jinja = engines["Jinja2"]
-
     try:
         # By default, we do both Jinja2 templating and Markdown
         # processing on the file, to make it easy to use both Jinja2
@@ -169,6 +168,8 @@ def render_markdown_path(
         else:
             raise e
 
+    API_ENDPOINT_NAME = context.get("API_ENDPOINT_NAME", "")
+    markdown_string = markdown_string.replace("API_ENDPOINT_NAME", API_ENDPOINT_NAME)
     html = md_engine.convert(markdown_string)
     rendered_html = jinja.from_string(html).render(context)