Make repo link a separate component to allow using GitHub buttons

[epoberezkin]

Feature request

What problem does this feature solve?

Replacing repo link in Navbar with some GitHub button (star, fork. etc.)

What does the proposed API look like?

2 options possible:

1. No api change for theme users
2. theme config allowing to chose GitHub button parameter (can be a separate change)

How should this be implemented in your opinion?

1. Make repo link in NavLinks a separate component (say RepoLink - let me know what name you prefer) so that inherited theme can redefine it to use GitHub buttons (star, fork, etc.).
2. Additionally, add GitHub button component and use it based on configuration.

Are you willing to work on this yourself?

Yes for option 1, possibly later on option 2.