[2pt] Document luajit-gdb.py

The luajit-gdb.py isn't mentioned in doc. That's no good.

SME: @igormunkin

[@veod32]:
Introduced in release 2.4.1.
Summary from the release notes (LuaJIT section):

Introduce luajit-gdb.py extension with commands for inspecting LuaJIT internals. The extension obliges one to provide gdbinfo for libluajit, otherwise loading fails. The extension provides the following commands:

• lj-arch dumps values of LJ_64 and LJ_GC64 macro definitions
• lj-tv dumps the type and GCobj info related to the given TValue
• lj-str dumps the contents of the given GCstr
• lj-tab dumps the contents of the given GCtab
• lj-stack dumps Lua stack of the given lua_State
• lj-state shows current VM, GC and JIT states
• lj-gc shows current GC stats

More details in this cover letter

ToDo

• The luajit-gdb.py script can be mentioned in some documentation section related to LuaJIT -- research where.
• As for the script usage, there is an internal help that can be started from GDB (GNU Debugger). The help contains all the necessary info for developers, and there is no need to duplicate this in the Tarantool documentation.

Start by reviewing src/luajit-gdb.py and the LuaJIT section of the documentation to find an appropriate place for the reference. Use the release notes and linked cover letter for context, then add a mention of the script and its purpose without duplicating its internal GDB help. Done means the documentation points developers to luajit-gdb.py and explains what it provides.