primary entry page issues

[mattgarrish]

This PR addresses the following three issues on the primary entry page (pep):

• The primary entry page may be under specified #329 - that we should have a dedicated section to explain the pep
• manifest must be embedded in primary entry page #333 - that the manifest must be embedded in the pep
• primary entry page must be added to reading order when reading order not specified #334 - that the pep has to be added to the default reading order when the reading order isn't specified

Feedback on whether I've captured the intent of the page correctly welcome.

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[iherman]

There may be some details to improve, but it would be important to merge this PR and take it from there. It covers a major missing editorial aspect of the document which may be the source of misunderstandings.