docs: Add BibTeX for people to cite Kotaemon in their work

[lone17]

Description

• Add BibTeX for people to cite Kotaemon in their work

Type of change

• New features (non-breaking change).
• Bug fix (non-breaking change).
• Breaking change (fix or feature that would cause existing functionality not to work as expected).
• Documentation and miscellaneous

Checklist

• I have performed a self-review of my code.
• I have added thorough tests if it is a core feature.
• There is a reference to the original bug report and related work.
• I have commented on my code, particularly in hard-to-understand areas.
• The feature is well documented.

[taprosoft]

LGTM