Topics: final updates and add blog post announcement

[harding]

(Only review final three commits. This is based on a version of #244 that has itself been rebased on master.)

This makes three changes to the website:

• Updates the topic indexes with the latest links to our content
• Activates the change to the site header, (1) moving Newsletters and Blog to Publications and (2) putting Topics in the header
• Adds a blog post announcing the topics index

[jnewbery]

ACK cd1546b

I've reviewed the last three commits and checked all links. Great stuff as always, @harding. Thanks for your hard work on this.

[jnewbery]

rebased on master: https://github.com/bitcoinops/bitcoinops.github.io/compare/9510ba8ef3debb8708fd97555d9ac405906f3190..ba7b696e998b9d19bb37db520dee711742692852

[jnewbery]

rebased on master: https://github.com/bitcoinops/bitcoinops.github.io/compare/4ca92c6ec70564cbd85441fb4b396a71849a6ba0..451bf047d592e533367334e52926b6938d6fcc3a