Update "How to access etcd" page: add prereq, add h2 header, issue 790

[reylejano]

Fix issue: https://github.com/etcd-io/website/issues/790

This PR adds the following:

• Add prerequisites (etcdctl is required)
• Define all variables used in the examples in a bulleted list (in this case, $ENDPOINTS, foo and `"Hello world"

Add sub-sections (## H2 headings) for:

• Pre-requisites
• Write to an etcd cluster
• Read from an etcd cluster