[DOCS] Environment variables page is outdated and misleading

[Erikas]

https://openobserve.ai/docs/environment-variables/ is outdated/misleading and does not represent current status of the project (based on breaking changes disclaimer in https://github.com/openobserve/openobserve/releases/tag/v0.10.0)

1. ZO_META_STORE does not contain anything related to mysql.
2. nats not mentioned at all.
3. Some information is misleading? etcd will not be used by default for cluster mode. 🤔

For local mode, OpenObserve use sqlite as the metadata store.
For cluster mode, OpenObserve use etcd as the metadata store.

Additionally, I am requesting you to update this page and specify which are needed for cluster version, which are needed for standalone version and which are common for both. 🙏 Preferably, an additional column specifying for what version variable should be used.

Also, could you please fix (or update docs if that's already the case) the default values of these?

To my belief, setting only ZO_DATA_DIR to something like /var/lib/openobserve/ should be enough for ZO_DATA_DB_DIR, ZO_DATA_WAL_DIR, ZO_DATA_STREAM_DIR and ZO_DATA_CACHE_DIR to default to /var/lib/openobserve/<dir> rather than remain ./data/openobserve/<dir>. 🤔 Their defaults should be updated (if that's the case) to $ZO_DATA_DIR/<dir>

[Erikas]

Related: #3310 (comment)

[Juan Calderon-Perez]

This variable is also missing from the docs ZO_INGEST_FLATTEN_LEVEL, it was introduced in #3218

This issue should probably go in https://github.com/openobserve/openobserve-docs

@prabhatsharma @hengfeiyang Updates to docs are urgent, kind of hard to be checking the source code to see which new ENV's are available

[Prabhat Sharma]

I am working on updating it. Already made some updates. I'll try to finish it as soon as possible.

[Prabhat Sharma]

Closing this as all variables have been covered. We will keep improving the minor ones.