Document option changes in the HTML documentation

**Description of the desired feature**

I'll take the recent changes in the `psconvert` module as an example.

In PR #5583, some modifiers of the **-A** option are moved to the **-I** option. The changes are handled in a compatible way, so that old scripts using `-A+m2c` still works.

However, users may need to read and understand old scripts that were written by someone else a few years ago. They definitely would be very confused when they see `-A+m2c` in the old scripts, but don't see such usage in the GMT HTML documentation.

I think we should use [Sphinx directives `versionadded`, `versionchanged` and `deprecated`](https://www.sphinx-doc.org/en/master/usage/restructuredtext/directives.html#paragraph-level-markup) to document these changes.

For the `psconvert` **-A** and **-I** changes, we may add a short description, like:

```
**-A**
Description of **-A** option.

.. versionchanged: 6.3.0
Modifies +m, +g, +p, et al. are moved to the **-I** option.

**-I**
Description of **-I** option.

.. versionchanged: 6.3.0
Add modifies +m, +g, +p, et al.
```

Such change history is definitely more useful than the changelog for each release.