Organization of Understand section

[sinaeftekhar]

As suggested in another issue(issue #95), "Understand" section needs some improvement. These points are being suggested:

• Change title “About” to “Overview”(it clarifies more about the content of this page).
• Change title “Features” to “Terminology”(or something similar), since features confuses user with the broadly used term feature throughout the documentation.
• move each of sections in spaces , components, and participants (for example processes, assemblies, etc in spaces), to a separate sub-category.
• Use internal/external links whenever it can be utilized (ones that may help user understand those terms).