Outdated links in Task 1 and Task 3

[KulaginVladimir]

Some links in Task 1 are outdated:

FESTIM-workshop/tasks/task01.ipynb

Lines 127 to 128 in 987d27c

	"> All units in FESTIM as SI (apart for activation energies that are in eV)\n",
	"> To check what unit is expected by FESTIM, check the documentation. [Here](https://festim.readthedocs.io/en/latest/api/festim.materials.html#festim.materials.material.Material) is the reference for the `Material` class"

FESTIM-workshop/tasks/task01.ipynb

Line 176 in 987d27c

"FESTIM provides plenty of boundary conditions (see [Dirichlet BCs](https://festim.readthedocs.io/en/latest/api/festim.boundary_conditions.dirichlets.html#festim-boundary-conditions-dirichlets-package) and [Fluxes](https://festim.readthedocs.io/en/latest/api/festim.boundary_conditions.fluxes.html)).\n",

I also missed this one in #46. Sorry about that.

FESTIM-workshop/tasks/task01.ipynb

Line 155 in 987d27c

">For heat transfer simulations, the `HeatTransferProblem` can be used instead. See [Heat transfer simulation](https://github.com/festim-dev/FESTIM-workshop/blob/main/tasks/task6.ipynb)"

and in Task 3:

FESTIM-workshop/tasks/task03.ipynb

Line 349 in 987d27c

" \n",

[RemDelaporteMathurin]

The links to the docs should be easy to fix.
To fix the link to the image in Task 3 one would need to rerun the notebook to obtain it 😄