How to decode ARMv7 instructions?
The ARMv7-M reference manual does not have a decode lookup table, but the encoding for each instruction is listed.

In the specific manual you have linked, you will find this in the section A6.7 - Alphabetical list of ARMv7-M Thumb instruction.

This contains - as stated - a list of every instruction, and their corresponding binary encoding. For example, page A6-16 has the binary encoding for the ADC (immediate) instruction. We can see that it starts with 11110... and spans two words.

The problem is going "backwards". The manual does indeed contain all the necessary information, but it is difficult to search. The disassembler has the data structures in place to do that for you, and if you want a table and do it manually, I suggest looking at the source code files for an ARM disassembler, for example GNU binutils.