Is there a word for a portion of information presented at once?

The initial chunk of information may be referred to as an abstract/synopsis/anecdotes. Later, when the users want more information, they can read the full description of the article.

• abstract (noun)

  Make a written summary of (an article or book)

  [ODO]

• synopsis

  A brief summary or general survey of something

  [ODO]

The particular web page that you have linked presents an abstract/ a synopsis at the top and expands on it as per user's demand.

I'd use anecdotes rather cautiously in this context as it mainly deals with real life incidents.

Depending on the nature of the information, point may be appropriate. Among its many meanings is

a single or separate article or item, as in an extended whole; a detail or particular

Source: Dictionary.com

To use it in your case,

A page on this site consists of multiple points, which will be presented to the user one at a time.
When writing your own points, it could be helpful to consider how much information you wish to encourage a reader to absorb before asking for more.

Points has many meanings, so at times it might be less clear or ambiguous but it can be used to address singular concepts being addressed.

You might consider the term "packet". It has a meaning related to electronic data transfer. It might possibly be too technical for your application though.

a short fixed-length section of data that is transmitted as a unit in an electronic communications network

Slide is commonly used, and borrows from the PowerPoint concept, which itself borrowed from the slide projector.