docs: Add reference for Thumb2 inline assembler.

docs/pyboard/tutorial/assembler.rst

docs/reference/asm_thumb2_arith.rst

docs/reference/asm_thumb2_compare.rst

docs/reference/asm_thumb2_directives.rst

docs/reference/asm_thumb2_float.rst

docs/reference/asm_thumb2_hints_tips.rst

docs/reference/asm_thumb2_index.rst

docs/reference/asm_thumb2_label_branch.rst

docs/reference/asm_thumb2_ldr.rst

docs/reference/asm_thumb2_logical_bit.rst

docs/reference/asm_thumb2_misc.rst

docs/reference/asm_thumb2_mov.rst

docs/reference/asm_thumb2_stack.rst

docs/reference/asm_thumb2_str.rst