Add documentation index for doc/ as README.md, and small docs fixes (#16253)

doc/README.md

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+# Documentation
+
+This directory contains mostly reference documentation for the Luanti engine.
+For a less prescriptive and more guiding documentation, also look at:
+https://docs.luanti.org
+
+Note that the inner workings of the engine are not well documented. It's most
+often better to read the code.
+
+Markdown files are written in a way that they can also be read in plain text.
+When modifying, please keep it that way!
+
+Here is a list with descriptions of relevant files:
+
+## Server Modding
+
+- [lua_api.md](lua_api.md): Server Modding API reference. (Not only the Lua part,
+    but also file structure and everything else.)
+    If you want to make a mod or game, look here!
+    A rendered version is also available at <https://api.luanti.org/>.
+- [builtin_entities.md](builtin_entities.md): Doc for entities predefined by the
+    engine (in builtin), i.e. dropped items and falling nodes.
+
+## Client-Side Content
+
+- [texture_packs.md](texture_packs.md): Layout and description of Luanti's
+    texture packs structure and configuration.
+- [client_lua_api.md](client_lua_api.md): Client-Provided Client-Side Modding
+    (CPCSM) API reference.
+
+## Mainmenu scripting
+
+- [menu_lua_api.md](menu_lua_api.md): API reference for the mainmenu scripting
+    environment.
+- [fst_api.txt](fst_api.txt): Formspec Toolkit API, included in builtin for the
+    main menu.
+
+## Formats and Protocols
+
+- [world_format.md](world_format.md): Structure of Luanti world directories and
+    format of the files therein.
+    Note: If you want to write your own deserializer, it will be easier to read
+    the `serialize()` and `deSerialize()` functions of the various structures in
+    C++, e.g. `MapBlock::deSerialize()`.
+- [protocol.txt](protocol.txt): *Rough* outline of Luanti's network protocol.
+
+## Misc.
+
+- [compiling/](compiling/): Compilation instructions, and options.
+- [ides/](ides/): Instructions for configuring certain IDEs for engine development.
+- [developing/](developing/): Information about Luanti development.
+    Note: [developing/profiling.md](developing/profiling.md) can be useful for
+    modders and server owners!
+- [android.md](android.md): Android quirks.
+- [direction.md](direction.md): Information related to the future direction of
+    Luanti. Commonly referred to as the roadmap document.
+- [breakages.md](breakages.md): List of planned breakages for the next major
+    release, i.e. 6.0.0.
+- [docker_server.md](docker_server.md): Information about our Docker server
+    images in the ghcr.