[feat][doc] add instructions of environment variables in pulsar_env.sh

### Search before asking

- [X] I searched in the [issues](https://github.com/apache/pulsar/issues) and found nothing similar.

### What issue do you find in Pulsar docs?

@asafm
While trying to setup docker compose to run on my mac, it fails on OOM.
I tried searching for a listing of environment variables, so I can configure the memory for the JVM but couldn’t find one.
@hangc0276 helped to understand its PULSAR_MEM . I searched for it and couldn’t find it using Search bar.
I think we should have environment variables listed under Administration.

@Anonymitaet
Do we need to add explanations for all the environment variables in [pulsar_env.sh](https://github.com/apache/pulsar/blob/master/conf/pulsar_env.sh#L21) to docs?

Asaf Mesika
IMO all besides JAVA_HOME. We don’t have time even just the memory one will be great.

### What is your suggestion?

Add instructions of environment variables in pulsar_env.sh

### Any reference?

_No response_

### Are you willing to submit a PR?

- [ ] I'm willing to submit a PR!

Start with conf/pulsar_env.sh, especially the environment variables around line 21, and review the Administration documentation. Document PULSAR_MEM and determine whether the remaining variables should also be covered; the work is done when users can find clear configuration instructions through the documentation search.

Written by the indexing model from the issue text.