Document tools/macro_builder and tidy up serve_header.py

tools/macro_builder generates the NLOHMANN_JSON_EXPAND,
NLOHMANN_JSON_GET_MACRO and NLOHMANN_JSON_PASTE* macros in
macro_scope.hpp, but nothing referred to it. Add a README that explains
what it generates, how to run it and where the output goes, and which
dependent tables (NLOHMANN_JSON_DOUBLE_PASTE, NLOHMANN_JSON_TYPE_BODY)
are maintained by hand. Point to it from a comment above
NLOHMANN_JSON_EXPAND. The generator itself is unchanged; following the
README reproduces the header byte for byte.

In serve_header.py, drop the LGTM suppression (LGTM.com shut down in
2022), replace the """.""" placeholder docstrings with real ones, and
import socket and ssl at module level. DualStackServer.server_bind uses
socket, which was only imported under __main__; when the module was
imported instead, the NameError was swallowed and IPV6_V6ONLY was not
cleared.

The header change is a comment only; behavior, API and ABI are
unchanged.

Part of #5717

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tools/macro_builder/README.md

@@ -0,0 +1,43 @@
+# macro_builder
+
+Generates the argument-counting macros behind the `NLOHMANN_DEFINE_TYPE_*` and `NLOHMANN_DEFINE_DERIVED_TYPE_*`
+macros in [`include/nlohmann/detail/macro_scope.hpp`](../../include/nlohmann/detail/macro_scope.hpp):
+
+- `NLOHMANN_JSON_EXPAND`
+- `NLOHMANN_JSON_GET_MACRO`, which selects a macro by the number of its arguments (64 slots)
+- `NLOHMANN_JSON_PASTE`, which calls a function-like macro for each member, and its helpers `NLOHMANN_JSON_PASTE2`
+  to `NLOHMANN_JSON_PASTE64`
+
+The number of slots (`max_args` in [`main.cpp`](main.cpp)) sets the member limit of these macros.
+`NLOHMANN_JSON_PASTE` takes the function as its first argument, so 64 slots allow 63 members.
+
+## Usage
+
+From the project root:
+
+```shell
+c++ -std=c++11 tools/macro_builder/main.cpp -o macro_builder
+./macro_builder
+```
+
+1. In `include/nlohmann/detail/macro_scope.hpp`, replace the lines from `#define NLOHMANN_JSON_EXPAND( x ) x` to the
+   `#define NLOHMANN_JSON_PASTE64(...)` line with the output, without its trailing empty line.
+2. Run `make amalgamate`. It updates `single_include/nlohmann/json.hpp` and runs `make pretty`, which indents the
+   continuation lines of `NLOHMANN_JSON_PASTE` that the tool writes unindented.
+
+With an unchanged `main.cpp`, these steps reproduce the header byte for byte.
+
+## Maintained by hand
+
+The tool does not generate everything that depends on the number of slots. When changing `max_args`, also update:
+
+- the `NLOHMANN_JSON_DOUBLE_PASTE` table right after the generated block (`NLOHMANN_JSON_DOUBLE_PASTE` and
+  `NLOHMANN_JSON_DOUBLE_PASTE3`, `NLOHMANN_JSON_DOUBLE_PASTE5`, ..., `NLOHMANN_JSON_DOUBLE_PASTE63`), which the
+  `*_WITH_NAMES` macros use
+- the slot table of `NLOHMANN_JSON_TYPE_BODY`, which chooses between the implementations for zero members and for one
+  or more members
+- the documented limit of 63 members in `docs/mkdocs/docs` and the tests at that limit in
+  `tests/src/unit-udt_macro.cpp`
+
+Both tables pass one macro name per slot to `NLOHMANN_JSON_GET_MACRO`, so they need exactly as many entries as it has
+slots.