Provide installable man pages

make docs: generate Markdown and man pages in doc/
make install-docs: install man pages to be usable directly

Docs are generated from the ezdoc files in doc/src/.

doc/src/guide/architecture.ezdoc

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+::: Architecture
+
+Cowboy is a lightweight HTTP server.
+
+It is built on top of Ranch. Please see the Ranch guide for more
+information.
+
+:: One process per connection
+
+It uses only one process per connection. The process where your
+code runs is the process controlling the socket. Using one process
+instead of two allows for lower memory usage.
+
+Because there can be more than one request per connection with the
+keepalive feature of HTTP/1.1, that means the same process will be
+used to handle many requests.
+
+Because of this, you are expected to make sure your process cleans
+up before terminating the handling of the current request. This may
+include cleaning up the process dictionary, timers, monitoring and
+more.
+
+:: Binaries
+
+It uses binaries. Binaries are more efficient than lists for
+representing strings because they take less memory space. Processing
+performance can vary depending on the operation. Binaries are known
+for generally getting a great boost if the code is compiled natively.
+Please see the HiPE documentation for more details.
+
+:: Date header
+
+Because querying for the current date and time can be expensive,
+Cowboy generates one `Date` header value every second, shares it
+to all other processes, which then simply copy it in the response.
+This allows compliance with HTTP/1.1 with no actual performance loss.
+
+:: Max connections
+
+By default the maximum number of active connections is set to a
+generally accepted big enough number. This is meant to prevent having
+too many processes performing potentially heavy work and slowing
+everything else down, or taking up all the memory.
+
+Disabling this feature, by setting the `{max_connections, infinity}`
+protocol option, would give you greater performance when you are
+only processing short-lived requests.