[Contribution docs] Add GitPOAP Badge to Display Number of Minted Git…

…POAPs for Contributors (#3343)

## Issue Addressed - N/A

## Proposed Changes

Adding badge to contribution docs that shows the number of minted GitPOAPs

## Additional Info

Hey all, this PR adds a [GitPOAP Badge](https://docs.gitpoap.io/api#get-v1repoownernamebadge) to the contribution docs that displays the number of minted GitPOAPs for this repository by contributors to this repo.

You can see an example of this in [our Documentation repository](https://github.com/gitpoap/gitpoap-docs#gitpoap-docs).

This should help would-be contributors as well as existing contributors find out that they will/have received GitPOAPs for their contributions.

CC: @colfax23 @kayla-henrie

Replaces: #3330

Co-authored-by: Michael Sproul <micsproul@gmail.com>

CONTRIBUTING.md

@@ -1,4 +1,5 @@
 # Contributors Guide
+[![GitPOAP badge](https://public-api.gitpoap.io/v1/repo/sigp/lighthouse/badge)](https://www.gitpoap.io/gh/sigp/lighthouse)
 
 Lighthouse is an open-source Ethereum 2.0 client. We're community driven and
 welcome all contribution. We aim to provide a constructive, respectful and fun

book/src/contributing.md

@@ -1,6 +1,7 @@
 # Contributing to Lighthouse
 
 [![Chat Badge]][Chat Link]
+[![GitPOAP Badge](https://public-api.gitpoap.io/v1/repo/sigp/lighthouse/badge)](https://www.gitpoap.io/gh/sigp/lighthouse)
 
 [Chat Badge]: https://img.shields.io/badge/chat-discord-%237289da
 [Chat Link]: https://discord.gg/cyAszAh