Minor clean up for documentation

[PipKat]

Description of the modifications

Minor improvements for doc:

• In the calc_functions.rst file, the equation for NodeToElem() could be better formatted to look like those for other calculator functions.
• Consider converting the image /_static/UM-C7-12.png to SVG file for possibly making the display better in dark mode.
• Fix display issues in the rendered doc. The Object API reference, which is imported, has truncated descriptions on its landing page. Also, on the landing page for the Session class, there are some method descriptions that are truncated.

Useful links and references

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