#849 PLCgen S7 docs: update/improve/fix documentation.

Changes:

• 'Convert enumerations' option now has an 'auto' value as well.
• For S7-1500, we use DINT (32-bit) and not LINT (64-bit) by default for integer values.
• Make 'standard compliance' examples, as it is likely not complete.
• Explain steps are for TIA Portal v15.
• Explain to import all '*.xml' tag files that exist.
• 'CONSTS.xml' is now named 'CONSTANTS.xml'.
• Fix syntax error for a button.
• No need to enter input/output addresses, as PLCgen generates them.
• No need to make outputs manually, as PLCgen generates them.
• Removed section for configuring inputs/outputs; PLCgen generates them.
• Add that more steps may be needed on 'Load preview' window.
• There is no 'state0' variable anymore. Unfold the 'Static' item now.

Closes #849 (closed)