More schem updates

[uecasm]

Closes #1022

Changes proposed:

• Imports the changes from the old PR to clarify some schematics info
• Fixes some incorrect links from the conversion
• Tweaks a bit more stuff for extra clarity
• ~Allows you to specify override text in an item link, e.g. {% item name="placeholderblocks" %}light placeholder{% /item %}~
• Fixes incorrectly referring to the Placeholder block as Solid Placeholder.