Add operation modes definition to glossary list

[Fashander]

What should be done?

In the glossary list https://github.com/FerretDB/FerretDB/blob/main/website/docs/reference/glossary.md, we need to include information about FerretDB Operation modes.

DoD:

• Add a definition for Operation modes in FerretDB under section "O"
• Link to the Operation modes page at https://docs.ferretdb.io/configuration/operation-modes/

See the writing guide for help: https://docs.ferretdb.io/contributing/writing-guide/

[rohitkbc]

Hi @Fashander 👋, Can you assign this task to me once it is ready?