Make /merch page findable via menu botton/header/footer links

[shushugah]

Currently /merchandise is live on website, but not linked anywhere. Make it accessible, by updating the header and footers.

Code:

• The header depends on both site.links and site.header_links in Jekyll config file

Acceptance criteria

• Replace /chapters link in the header_links with /Merch
• Make sure it looks good in mobile view and desktop view
• Any other recommendations to make menu button/header less cluttered?