Upgrade Forest and Ridge plots documentation

#### issue

+ Update the function's description:

- Add more information about ridge plots.
- Change the term 'traces' for 'models.'
- The forest plot's description should specify that the central points are the posterior means.

#### Possible solution:

```
Generate forest and ridge plots to compare distributions from a model or list of models.

The `forestplot` option generates credible intervals, where the central points are the estimated posterior means,
the thick lines are the central quartiles, and the thin lines represent the $100\times$(`hdi_prob`)% HDI intervals.
The `ridgeplot` option generates KDE posterior densities truncated at 0.94 probability.

Additionally, the function displays effective sample sizes (ess) and Rhats to visualize the convergence diagnostic.
```

#### issue

+ update `combine_dims` argument: The expression See the [this section](https://arviz-devs.github.io/arviz/user_guide/plots_arguments_guide.html#common-combine-dims) for usage examples." has an extra "**the**."

+ update `coords` argument: Add the expression: "See [this section](https://arviz-devs.github.io/arviz/user_guide/plots_arguments_guide.html#common-combine-dims)," at the end of the description of the argument.

+ Add extra examples to specify when the plot can be used as a convergence diagnostic tool.

### Possible examples:

```
Convergence diagnostic forest plot

.. plot::
:context: close-figs

>>> axes = az.plot_forest(non_centered_data,
>>> combined=False,
>>> ess = True,
>>> figsize=(9, 7))
>>> axes[0].set_title('Estimated theta for 8 schools model')

Convergence diagnostic Ridge plot

.. plot::
:context: close-figs

>>> axes = az.plot_forest(non_centered_data,kind='ridgeplot',
>>> combined=True,
>>> ess = True,
>>> r_hat = True,
>>> figsize=(9, 7))
>>> axes[0].set_title('Estimated theta for 8 schools model')
```

Start with the documentation for az.plot_forest and the linked common-combine-dims section. Update the forest and ridge plot descriptions, correct the combine_dims and coords wording, and add the proposed convergence-diagnostic examples; done means the rendered documentation reflects these descriptions, links, and examples.

Written by the indexing model from the issue text.