Update the contact link in the documentation

[leouieda]

Description:

Update the contact link in the documentation pages to https://www.fatiando.org/contact/. This is the new page we have that explains all the ways to reach us. It's more stable than the forward we had to our Slack channel (contact.fatiando.org). The link needs to be replaced in the doc/index.rst file of our documentation.

Apply to:

• boule
• harmonica
• pooch
• verde

Further instructions:

• Start by opening Pull Requests on each repository listed above.
• Optionally, we can open Issues on each repository if further discussion specific to that repository is needed.
• Mention this Issue on every Issue or Pull Request opened on each opened: Related to fatiando/community#XX
• Check-off the repository on the list above once the Pull Request is merged.
• Close this issue when all items are checked-off.

We want your help!

We know that maintenance tasks are very demanding, so we don't expect a single person to tackle this issue by themselves. Any help is very welcomed, so please comment below that you want to take care of the changes on any repository and we will assign it to you.

[leouieda]

Done in all packages.