docs: add missing ::regclass cast error to troubleshooting

[aditya3799]

What does this PR do?

Adds a new section to the Troubleshooting guide (troubleshooting.mdx) to address the function does not exist error that occurs when users omit the ::regclass cast during schema registration.

Why is this change needed?

When new users follow the manual schema registration examples (e.g., in the Quickstart), it's very common to accidentally pass table names as plain strings instead of OIDs. This results in the following Postgres error:

ERROR:  function graph.add_table(unknown, unknown) does not exist
HINT:  No function matches the given name and argument types. You might need to add explicit type casts.

Closes #17