This commit expands the language about Executive Sponsors in Community
Initiatives by adding a section that clarifies how the Executive Sponsor
is selected, what the expectations are between the Lead and the Sponsor,
and other requirements for the Initiative (e.g. an Executive Sponsor
must be attached to an Initiative in order for it to proceed).

Additionally, I made minor changes to other parts of the document, such
as using more xref links to other pages in the Fedora documentation
site, adding a lead-in at the top of the Initiatives page, and other
English usage/mechanics edits.

These changes were worked on collaboratively between myself, @amoloney,
and @mattdm.

Closes #189.