[e.cash] Add mining section

Summary: Adding the mining section to the bottom of /get-cash

Test Plan:
preview the site and navigate to the mining section on /get-ecash
check everything looks good and link works
*note the link will be updated to new blog once thats up, issue created

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D14098