Show links to `previous` and `next` pages

[stephanie-anderson]

Description

Add buttons for easier navigation through docs at the bottom of each page (for previous and next pages), similar to this one here:

Source: https://docusaurus.io/docs/static-assets

Requirements

1. General:

• Despite the screenshot below, wording should be Previous and Next
• Buttons should render the page's title

2. Automatically generate previous and next buttons:

• The docs index page (/) does neither have previous nor next buttons
• The first child of a parent node should render the parent as its previous page
• The last child of a parent node should render the next sibling of its parent as next page
• If that parent does not have a next sibling, no next page link should be rendered

• If the page is a direct child of the index page (/), previous should point to the index page (/)

3. It should be possible to manually overwrite what previous and next should be for a given page. For example, for some JavaScript SDKs we have 3 different ways for users to install the SDK. We don't want to guide users from one install method to the others, but instead might guide them next to some configuration pages.