Add link to top page

[ahuglajbclajep]

This PR adds a link to the top page in the header.

[welcome[bot]]

Welcome! Congrats on your first pull request to The Slate Theme. If you haven't already, please be sure to check out the contributing guidelines.

[benbalter]

Per the contributing guidelines, if you are making visual changes, please include a screenshot of what the affected element looks like, both before and after.

[ahuglajbclajep]

We actually use this theme on our web page. https://prog-g.github.io

The only visual change is that the scroll bar is always displayed because the position of headings moved slightly depending on the presence or absence of the scroll bar. Therefore, it will be displayed like this on pages with low content.

Before:

After:

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.