[DOC] document download URL for core plugins

[rursprung]

What do you want to do?

• Request a change to existing documentation
• Add new documentation
• Report a technical problem with the documentation
• Other

Tell us about your request.
in most cases the core plugins can just be installed by running opensearch-plugin install ${pluginname}. however, if you're offline you need to have access to the ZIP file of the plugin and accordingly need to download it first.
for plugins hosted in other repositories this is not an issue, as they're (a) published as a github release and (b) usually published to maven central.

currently, the download URL for core plugins is not documented, thus it's not possible to just download them.

Version: n/a (all of them)

What other resources are available?
as discussed on slack with @reta & @saratvemulapalli who can provide the necessary input here

[rursprung]

@Naarcha-AWS: please don't assign this to me - I don't know the URL to be documented 😅

[reta]

@Naarcha-AWS please assign it to me, thank you!

[rursprung]

i now manually pieced together based on some code that the URL e.g. for my plugin is https://artifacts.opensearch.org/releases/plugins/analysis-phonenumber/2.18.0/analysis-phonenumber-2.18.0.zip - i guess it's always https://artifacts.opensearch.org/releases/plugins/${PLUGIN}/${OS_VERSION}/${PLUGIN}-${OS_VERSION}.zip?

[reta]

i guess it's always https://artifacts.opensearch.org/releases/plugins/${PLUGIN}/${OS_VERSION}/${PLUGIN}-${OS_VERSION}.zip?

@rursprung I am waiting till opensearch-project/opensearch-build#5188 is resolved so we would ensure no changes in the SNAPSHOT repo coordinates.