docs: update quickstart-server instructions

[MEllis-github]

Please describe the purpose of this pull request.
Is it to add a new feature? Is it to fix a bug?

When following the server quickstart instructions as is, $ docker compose up yields the following due to .env not being included in the repo except in the form of env.example. (Not a great out-of-the-box experience.) This PR adds a simple step on managing .env. It's not clear this is the best way to handle this, but at least takes care of the warnings and fatal error.

WARN[0000] The "MEMGPT_PG_USER" variable is not set. Defaulting to a blank string. 
WARN[0000] The "MEMGPT_PG_PASSWORD" variable is not set. Defaulting to a blank string. 
WARN[0000] The "MEMGPT_PG_DB" variable is not set. Defaulting to a blank string. 
WARN[0000] The "MEMGPT_SERVER_PASS" variable is not set. Defaulting to a blank string. 
WARN[0000] The "MEMGPT_PG_DB" variable is not set. Defaulting to a blank string. 
WARN[0000] The "MEMGPT_PG_USER" variable is not set. Defaulting to a blank string. 
WARN[0000] The "MEMGPT_PG_PASSWORD" variable is not set. Defaulting to a blank string. 
WARN[0000] The "OPENAI_API_KEY" variable is not set. Defaulting to a blank string. 
WARN[0000] The "MEMGPT_PG_USER" variable is not set. Defaulting to a blank string. 
WARN[0000] The "MEMGPT_PG_PASSWORD" variable is not set. Defaulting to a blank string. 
WARN[0000] The "MEMGPT_PG_DB" variable is not set. Defaulting to a blank string. 
env file /Users/mme/Documents/GitHub/MemGPT/.env not found: stat /Users/mme/Documents/GitHub/MemGPT/.env: no such file or directory

How to test
How can we test your PR during review? What commands should we run? What outcomes should we expect?

Run the server-quick start steps. docker compose up should start the server as intended.

Have you tested this PR?
Have you tested the latest commit on the PR? If so please provide outputs from your tests.

Yes, following the steps produced the intended result of the original quickstart instructions.

Related issues or PRs
Please link any related GitHub issues or PRs.

Is your PR over 500 lines of code?
If so, please break up your PR into multiple smaller PRs so that we can review them quickly, or provide justification for its length.

No

Additional context
Add any other context or screenshots about the PR here.

[sarahwooders]

lgtm! thank you so much!