Add an introductory section to /tech-leads

[mrice]

Currently the intro is in index.md, which should probably be renamed to README.md.

The purpose of the introductory section is to give an overview of the section for people who are tech leads and might be interested in getting coaching through the Tech Lead Network. (There is a different section of the site for people interested in being coaches, which will be the subject of a different issue momentarily.)

Part of the goal of an overview is to state the factual contents of the section, which is a fairly straightforward summary of the contents in the section.

The second part is to encourage readers to go beyond the introductory section with some compelling quotes and vision for why reading more of this, and possibly signing up for the network, could be worth their time investment.