CI: update docs and reset tool for checksums

[EZoni]

The major part of this PR is about updating the docs so that it is a bit easier for developers to connect the section on testing with the section on checksums.

Here's a couple of screenshots showing the new content organization for the testing and checksums sections:

As part of this PR, I also update the tool that we have to reset checksums locally based on the Azure output. The update is necessary due to a change in #5297, as noted in #5297 (comment), given that new checksum files are now displayed as follows:

New checksums file test_2d_langmuir_multi.json:
{
  "lev=0": {
    "Bx": 0.0,
    "By": 5.726296856755232,
    "Bz": 0.0,
    "Ex": 3751589134191.326,
    "Ey": 0.0,
    "Ez": 3751589134191.332,
    "jx": 1.0100623329922576e+16,
    "jy": 0.0,
    "jz": 1.0100623329922578e+16
  },
  "electrons": {
    "particle_momentum_x": 5.668407513430198e-20,
    "particle_momentum_y": 0.0,
    "particle_momentum_z": 5.668407513430198e-20,
    "particle_position_x": 0.6553599999999999,
    "particle_position_y": 0.65536,
    "particle_weight": 3200000000000000.5
  },
  "positrons": {
    "particle_momentum_x": 5.668407513430198e-20,
    "particle_momentum_y": 0.0,
    "particle_momentum_z": 5.668407513430198e-20,
    "particle_position_x": 0.6553599999999999,
    "particle_position_y": 0.65536,
    "particle_weight": 3200000000000000.5
  }
}

as opposed to the old way

----------------
New file for test_2d_langmuir_multi:
{
  "lev=0": {
    "Bx": 0.0,
    "By": 5.726296856755232,
    "Bz": 0.0,
    "Ex": 3751589134191.326,
    "Ey": 0.0,
    "Ez": 3751589134191.332,
    "jx": 1.0100623329922576e+16,
    "jy": 0.0,
    "jz": 1.0100623329922578e+16
  },
  "electrons": {
    "particle_momentum_x": 5.668407513430198e-20,
    "particle_momentum_y": 0.0,
    "particle_momentum_z": 5.668407513430198e-20,
    "particle_position_x": 0.6553599999999999,
    "particle_position_y": 0.65536,
    "particle_weight": 3200000000000000.5
  },
  "positrons": {
    "particle_momentum_x": 5.668407513430198e-20,
    "particle_momentum_y": 0.0,
    "particle_momentum_z": 5.668407513430198e-20,
    "particle_position_x": 0.6553599999999999,
    "particle_position_y": 0.65536,
    "particle_weight": 3200000000000000.5
  }
}
----------------

To-do:

• Update docs
• Update tool

[ax3l]

Thank you, LGTM! :)