Remove old docs, rename docs_new to docs

It would be probably good idea to also remove tools/docgen.py and
asciidoc.mk, but asciidoc.mk is still mentioned under `cloud/doc`, which
may contain some useful info which we'll need at least to review before
removing.

PUBLISHED_FROM=faf454d4c52a2f07ea8ac084cf0bd11a0c9c9b3b

docs/build-options/disabling-flags.md

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+---
+title: Disabling flags
+---
+
+- `MG_DISABLE_HTTP_WEBSOCKET` disable HTTP + Websocket protocol support
+- `MG_DISABLE_HTTP_DIGEST_AUTH` disable HTTP Digest (MD5) authorization support
+- `MG_DISABLE_MQTT` disable MQTT support
+- `MG_DISABLE_SHA1` disable SHA1 support (used by Websocket)
+- `MG_DISABLE_MD5` disable MD5 support (used by HTTP auth)
+- `MG_DISABLE_JSON_RPC` disable JSON-RPC support
+- `MG_DISABLE_SOCKETPAIR` disable `mg_broadcast()` API
+- `MG_DISABLE_HTTP_KEEP_ALIVE` useful for embedded systems to save resources

docs/build-options/enabling-flags.md

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+---
+title: Enabling flags
+---
+
+- `MG_ENABLE_SSL` Enable OpenSSL support
+- `MG_ENABLE_IPV6` Enable IPV6 support
+- `MG_ENABLE_THREADS` enable `mg_start_thread()` API
+- `MG_ENABLE_MQTT_BROKER` enable MQTT broker
+- `MG_ENABLE_DNS_SERVER` enable DNS server
+- `MG_ENABLE_COAP` enable CoAP protocol
+- `MG_ENABLE_GETADDRINFO` enable `getaddrinfo()` in `mg_resolve2()`

docs/build-options/intro.md

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+---
+title: Build Options
+---
+
+Mongoose source code ships in a single .c file that contains functionality
+for all supported protocols (modules). Modules can be disabled at compile
+time which reduces executable size. That can be done by setting preprocessor
+flags. Also, some preprocessor flags can be used to tune internal Mongoose
+parameters.
+
+To set a preprocessor flag during compile time, use `-D <PREPROCESSOR_FLAG>`
+compiler option. For example, to disable both MQTT and JSON-RPC,
+compile the application `my_app.c` like this (assumed UNIX system):
+
+```
+  $ cc my_app.c mongoose.c -D MG_DISABLE_MQTT -D MG_DISABLE_JSON_RPC
+```

docs/build-options/items.json

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+{
+  "items": [
+    { "type": "markdown", "name": "intro.md" },
+    { "type": "markdown", "name": "enabling-flags.md" },
+    { "type": "markdown", "name": "disabling-flags.md" },
+    { "type": "markdown", "name": "platform-spec.md" },
+    { "type": "markdown", "name": "tunables.md" }
+  ]
+}

docs/build-options/platform-spec.md

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+---
+title: Platform specific
+---
+
+Mongoose tries to detect the target platform whenever possible, but in some cases you have
+to explicitly declare some peculiarities of your target, such as:
+
+- `MG_LWIP`: use LWIP networking stack
+- `MG_CC3200`: enable workarounds for the TI CC3200 target.
+- `MG_ESP8266`: enable workarounds for the ESP8266 target, add `RTOS_SDK` to specify the RTOS SDK flavour.

docs/build-options/tunables.md

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+---
+title: Tunables
+---
+
+- `MG_MALLOC`, `MG_CALLOC`, `MG_REALLOC`, `MG_FREE` allow to use custom
+  memory allocator, e.g. `-DMG_MALLOC=my_malloc`
+- `MG_USE_READ_WRITE` when set replaces calls to `recv` with `read` and `send` with `write`,
+  thus enabling to add any kind of file descriptor (files, serial devices) to an event manager.
+- `MG_SSL_CRYPTO_MODERN`, `MG_SSL_CRYPTO_OLD` - choose either "Modern" or "Old" ciphers
+  instead of the default "Intermediate" setting.
+  See [this article](https://wiki.mozilla.org/Security/Server_Side_TLS#Recommended_configurations) for details.