Revise/clean up "Being a Tech Lead" page

[mrice]

The topic Being a Tech Lead is currently documented in intro.md, which maybe should be renamed to something more obvious, like 'being-a-tech-lead.md', but there are more substantive issues too:

Some specific issues:

• The section needs an introduction with a bit of an overview
• We repeat Pat Kua's definition and then offer our own but don't really explain the different
• Better cohesion between the sections
• We may seriously want to consider breaking the section "What is Tech Lead Coaching" into it's own page