r3.info: Add JSON support

[NishantBansal2003]

Fixes: #5995

This PR adds JSON support to the r3.info module. The changes include:

1. Updated descriptions for all flags.
2. Added a format option with plain, shell, and json modes for output formatting.
3. For the -g and -r flags, the default formats remain shell and plain otherwise, to preserve backward compatibility.
4. When no flag is given, two additional formats—shell and json—are now supported.
5. For the -g and -r flags, plain and json output formats have been added.
6. For the -h flag, a json output format have been added.
7. Added tests covering each of the new formats.
8. Added Python examples to the documentation, aligning it with the existing r.info module.

---

Below are example outputs for each format:

• Example: shell output

layer=volume
date=Mon Jul 21 23:14:24 2025
mapset=PERMANENT
creator=nishantbansal
project=nc_spm_08_grass7
database=/grassdata
title=volume
units=none
vertical_units=units
timestamp="none"
maptype=raster_3d
ncats=0
datatype=DCELL
rows=100
cols=200
depths=50
cells=1000000
size=147591
ntiles=294
meansize=502
tilesize=31320
tilenumx=7
tilenumy=7
tilenumz=6
tiledimx=29
tiledimy=15
tiledimz=9
projection=Lambert Conformal Conic
zone=0
north=200
south=100
nsres=1
east=400
west=200
ewres=1
top=500
bottom=450
tbres=1
min=3
max=350
source1=""
source2=""
description="generated by r3.mapcalc"
comments="r3.mapcalc expression="volume = row() + col() + depth()" region="cur\rent""

• Example: json output

{
    "layer": "volume",
    "date": "Mon Jul 21 23:14:24 2025",
    "mapset": "PERMANENT",
    "creator": "nishantbansal",
    "project": "nc_spm_08_grass7",
    "database": "/grassdata",
    "title": "volume",
    "units": "none",
    "vertical_units": "units",
    "timestamp": null,
    "maptype": "raster_3d",
    "ncats": 0,
    "datatype": "DCELL",
    "rows": 100,
    "cols": 200,
    "depths": 50,
    "cells": 1000000,
    "size": 147591,
    "ntiles": 294,
    "meansize": 502,
    "tilesize": 31320,
    "tilenumx": 7,
    "tilenumy": 7,
    "tilenumz": 6,
    "tiledimx": 29,
    "tiledimy": 15,
    "tiledimz": 9,
    "projection": "Lambert Conformal Conic",
    "zone": 0,
    "north": 200,
    "south": 100,
    "nsres": 1,
    "east": 400,
    "west": 200,
    "ewres": 1,
    "top": 500,
    "bottom": 450,
    "tbres": 1,
    "min": 3,
    "max": 350,
    "source1": "",
    "source2": "",
    "description": "generated by r3.mapcalc",
    "comments": "r3.mapcalc expression=\"volume = row() + col() + depth()\" region=\"current\""
}

For flag-specific output, see the tests in test_r3_info.py included with this PR.

[petrasovaa]

Looks great, just couple small suggestions.