Update SL Dimensions SCD section to comply with the validation rules about docs.getdbt.com

[SOVALINUX]

Thanks for the quick improvement @mirnawong1 !
However let me reopen the issue with some feedback:

1. I see the change of metric_time in one place in the doc, however simple page search gives 17 more occurences - I'm sure many of them should be adjusted as well
2. Overall it would be fantastic to put there some working examples, not some direction of the yml from previous versions. It's still not clear what is required and with what name based on your examples. Let me be specific here:
   a) entity_key (required): An entity_key (or some sort of identifier) must be present. -- how I should interpret it? To have any entity of any type or having dimension with the name entity_key?
   b) valid_from and valid_to that are required: should I have 2 dimensions named exactly like these or should I mark them via validity_params: is_start/is_end?

Really sorry for bothering you with it. A bit on the background: I'm running multiple projects with DBT for 2+ years, maintaining a dbt fork and consider myself quite an experienced used, and I still can't deal with these SCD2 dimensions without looking into the metricflow code, that's why I decided to help community and open this doc improvement request.
I know that this is not a error debug channel, but here is the error I have (maybe it will give you a hint where it's needed to update the docs, so users do know how to deal with it :) ):
ERROR: Cannot process join to data set with alias subq_9 because it has a validity window set: ValidityWindowJoinDescription(window_start_dimension=TimeDimensionSpec(element_name='valid_from', entity_links=(), time_granularity=TimeGranularity.DAY, date_part=None, aggregation_state=None), window_end_dimension=TimeDimensionSpec(element_name='valid_to', entity_links=(), time_granularity=TimeGranularity.DAY, date_part=None, aggregation_state=None)), but source data set with alias subq_7 does not have a metric time dimension we can use for the window join!

[mirnawong1]

update - we've merged a PR #5409 which addressed this feedback - i really appreciate it!

[mirnawong1]

hey @SOVALINUX thanks so much for the detailed write up - no worries at all and i appreciate you providing more context and looking out for the community ✨ - I'm going to look into this next week and see how can make this clearer for you. will revert back and thank you again!

with regards to your error, I'm going to flag this to the team internally to see what they say!

[mirnawong1]

also wanted to mention that we're merging this pr today which may be a little related: https://github.com/dbt-labs/docs.getdbt.com/pull/5350/files#diff-fc3ac6b16bd26fe485beef2324ef300d38959f05522372a9e5eb9e2ea66677c1R230