fix: Update link format per RST specifications.

[smiling-watermelon]

See: https://docutils.sourceforge.io/docs/ref/rst/restructuredtext.html#hyperlink-references

To see the difference, open the "Preview" mode in diffs.

Note: I'd suggest changing name of the hyperlinks to be not the same as the values of the hyperlinks.
For example, instead of:

If you have questions about the SignNow API, please visit `https://help.signnow.com/docs <https://help.signnow.com/docs>`_ or email `[email protected] <mailto:[email protected]>`_.

Make it like:

If you have questions about the SignNow API, please visit our `Docs Help Center <https://help.signnow.com/docs>`_ or email `[email protected] <mailto:[email protected]>`_.