Extension for markdown should improve readability

It was hard to read this online.  This should enable links where
appropriate.

Unfortunately, I am not positive that this won't break some tool relying on the filename not having an extension.

See merge request !771

Handbook minor corrections

General minor changes.

Attempt to maintain meanings, unless meaning was changed by  same
phonetically similar word to intended word.

Removed one gender reference, some Oxford commas, italics for an
example, quotes for example.

See merge request !772

Attempt to maintain meanings, unless meaning was changed by  same
phonetically similar word to intended word.

Removed one gender reference, some Oxford commas, italics for an
example, quotes for example.

It was hard to read this online.  This should enable links where
appropriate.

Added darby's bio

now officially part of the team :)

See merge request !769

Added Darby's Picture

for her bio :)

See merge request !768

Changed the pricing model to per month and added "charged yearly"

looks like this now: ![image](https://gitlab.com/gitlab-com/www-gitlab-com/uploads/ddd9babf06aa0085ca21854573ee50b8/image.png)

Fixes #348

See merge request !764

pm pages

See merge request !761

Release manager - invisible hero.

See merge request !746

Minor edits to core team Web page

1. september -> September
2. Fix mailing list link

Closes #347

See merge request !763

1. september -> September
2. Fix mailing list link

Closes #347

Leanpub Enterprise has unfortunately closed.

See merge request !762