Add a sidebar link to the Docs repository as well

**Describe the problem you have/What new integration you would like**
I "fell by parachute" at https://esphome.io/projects/, and wanted to make a contribution to that page. However, it took me _a while_ to find the docs repo, given it's not linked easily from that page.

**Please describe your use case for this integration and alternatives you've tried:**
> This is a docs feature request; I guess it should have a slightly different template? :sweat_smile:

My suggestion would be to add the Docs repo to the sidebar's footer as well, like `Source Code / Docs Source`. Another idea is to add the Contributing page to that footer - which is where I found the docs repo, after searching for something mentioning "projects".

**Additional context**
This suggestion is mildly in line with the HA docs, which usually have a prominent footer linking directly to the page's GH version, and links to report / check issues related to that file. Obviously that's way more complex, but a simple link on the sidebar could help those willing to contribute to the docs.

Start from the ESPHome projects page at https://esphome.io/projects/ and locate the sidebar footer configuration in the Docs repository. Add a link to the Docs repository or Contributing page, then verify that the link appears in the sidebar footer and points to the intended destination.

Written by the indexing model from the issue text.