Postgres & SQLite questions surrounding the "Platform-specific dependencies" documentation

This issue has been migrated from [#11748](https://github.com/matrix-org/synapse/issues/11748).

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Some questions surrounding the ["Platform-specific dependencies" section of the documentation](https://matrix-org.github.io/synapse/latest/setup/installation.html#platform-specific-prerequisites). I'm curious for feedback on what people think of the below:

In our instructions, Fedora lists dependencies for postgres support (`libpq-devel`), whereas none of the other platforms do. OpenSUSE and Arch Linux explicitly install `sqlite3`, but none of the other platforms do.

1. We could leave out postgres client dependencies from these lists. After all, Synapse does not run with postgres by default. However, we do **strongly** encourage people do run Synapse with postgres in production, so users will likely need to install the `libpq-dev` library eventually anyways (required for building [psycopg2](https://pypi.org/project/psycopg2/)). Why not do it here where we have platform-specific instructions, rather than only having instructions for Ubuntu/Debian distros on the [Using Postgres page](https://matrix-org.github.io/synapse/latest/postgres.html)?
2. I believe `sqlite3` is only needed for opening and inspecting an sqlite database - not for operation of Synapse, even when running with SQLite as the database backend. Should we drop `sqlite3` from the list of suggested packages?