Update SEPIO page (User Guide)

[larrybabb]

Status of our alignment Basic description of

• what it is,
• organization of owl files,
• where our terms come from in sepio,
• customizations/extensions outside of sepio

Links to it.

[cbizon]

I have taken a swing at this, but would like others to review.

[bpow]

I made a few changes in 245bd83fb86, otherwise 👍 from my standpoint