docs: fix readFile signature and add Options subsection

[Ankit-Mohanty-23]

The readFile heading was missing the options parameter despite
the code example showing it accepts pos and size as options.

Changes:

• Updated heading from readFile(filename) to readFile(filename, options)
• Moved pos and size into a new Options subsection

This aligns readFile with the readBinaryFile pattern in the same file.

Closes #<448>