Restructure file tree of notebooks

[santisoler]

Move the index file for each section to their corresponding folder. Update the TOC and mentions of old directories in notebooks and Markdown files.

Create the following file structure for the notebooks folder:

notebooks
├── dc-resistivity
│   ├── fwd_dcr_1d.ipynb
│   ├── fwd_dcr_2d.ipynb
│   ├── fwd_dcr_3d.ipynb
│   ├── index.md
│   ├── inv_dcr_1d_files
│   │   └── app_res_1d_data.dobs
│   ├── inv_dcr_1d.ipynb
│   ├── inv_dcr_2d.ipynb
│   └── inv_dcr_3d.ipynb
├── frequency-domain-em
│   ├── fwd_fdem_1d.ipynb
│   ├── fwd_fdem_3d.ipynb
│   ├── fwd_fdem_cyl.ipynb
│   ├── index.md
│   └── inv_fdem_1d.ipynb
├── gravity
│   ├── fwd_gravity_anomaly_3d.ipynb
│   ├── fwd_gravity_gradiometry_3d.ipynb
│   ├── index.md
│   ├── inv_gravity_anomaly_3d.ipynb
│   └── weighting_strategies.ipynb
├── induced-polarization
│   ├── fwd_ip_2d.ipynb
│   ├── fwd_ip_3d.ipynb
│   ├── index.md
│   ├── inv_ip_2d.ipynb
│   └── inv_ip_3d.ipynb
├── joint-inversion
│   └── plot_inv_3_cross_gradient_pf.ipynb
├── magnetics
│   ├── fwd_magnetics_induced_3d.ipynb
│   ├── fwd_magnetics_mvi_3d.ipynb
│   ├── index.md
│   └── inv_magnetics_induced_3d.ipynb
├── pgi
│   ├── plot_inv_1_joint_pf_pgi_full_info_tutorial.ipynb
│   └── plot_inv_2_joint_pf_pgi_no_info_tutorial.ipynb
└── time-domain-em
    ├── fwd_tdem_1d.ipynb
    ├── fwd_tdem_3d.ipynb
    ├── fwd_tdem_fundamentals.ipynb
    ├── fwd_utem_3d.ipynb
    ├── index.md
    └── inv_tdem_1d.ipynb

10 directories, 36 files