other-entry

You can query for that parent page's entry model with a new ElementCriteriaModel.

{% set mainEntry = craft.entries.id(100).first() %}

{% if mainEntry %}
    {% set matrixField = mainEntry.matrixFieldHandle %}
    {% set firstContentBlock = matrixField.type('content').first() %}

    {% if firstContentBlock %}
        {{ firstContentBlock.myRichTextFieldHandle }}
    {% endif %}
{% endif %}

Instead of using the entry's ID as the link to that entry, you can also use other parameters to query for the entry model. Here's an example using slug and section:

{% set mainEntry = craft.entries.section('news').slug('main-news').first() %}