docs: fix formula notation and heading hierarchy in indexed merkle tree documentation

[savvar9991]

1. Replacing v with new_nullifier:

-low_nullifier_{\textsf{next_value}} > v
+low_nullifier_{\textsf{next_value}} > new_nullifier

This correction is important because:
Eliminates confusion with variable v which isn't used anywhere else

2.Replacing \ | with ||:

-new_nullifier > low_nullifier_{\textsf{value}} : && : ( new_nullifier < low_nullifier_{\textsf{next_value}} : \ | : low_nullifier_{\textsf{next_value}} == 0 )
+new_nullifier > low_nullifier_{\textsf{value}} : && : ( new_nullifier < low_nullifier_{\textsf{next_value}} : || : low_nullifier_{\textsf{next_value}} == 0 )

\ | is an escape sequence for a vertical bar in LaTeX/Markdown
|| is the correct logical OR operator in mathematical formulas
Incorrect operator could lead to misunderstanding of the algorithm logic
Ensures proper rendering of the formula in math mode

3. Changing heading level:

-#### Performance gains from subtree insertion
+### Performance gains from subtree insertion

Makes the heading hierarchy more logical
"Performance gains" is a subsection of "Batch insertions", so it should be h3 level

[savvar9991]

@TomAFrench Sorry for pinging, but could you please take a look at this PR when you have a moment?