broken links due to recent Kafka External Stream docs update

[jovezhong]

With the update of #351 from @chenziliang , there are a few dead/broken links

output of yarn run build

Exhaustive list of all broken anchors found:
- Broken anchor on source page path = /enterprise-v2.8:
   -> linking to /proton-kafka#_tp_time
- Broken anchor on source page path = /external-stream:
   -> linking to /proton-kafka#limitations
- Broken anchor on source page path = /faq:
   -> linking to /proton-kafka#multi_col_read
- Broken anchor on source page path = /v1-release-notes:
   -> linking to /proton-kafka#multi_col_read
   -> linking to /proton-kafka#single_col_write