More detailed System tables and system view descriptions

[Frosendroska]

• Come up with a better ordering: Now the features are sorted in an alphabetical order. Probably the better way is to sort it by popularity.
• Examples: Add the reasonable examples for some features.
• Column-wise description: Which columns have meaningful data maybe makes more sense than which tables.
• Better links: For some features the link can be different. For example, for pg_index the better documentation is in pg_indexes