add ffe server sdk landing page and stubs for each language

[leoromanovsky]

What does this PR do? What is the motivation?

The FFE team is launching server SDKs in several languages this week (python, go, nodejs, java, ruby). Engineers that were DRIs for the projects are writing language specific documentation pages:

• https://github.com/DataDog/documentation/pull/33194
• https://github.com/DataDog/documentation/pull/33165

We need a landing page for server SDKs to explain common concept.s

Merge instructions

Merge readiness:

• [ ] Ready for merge

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Additional notes

[github-actions[bot]]

Preview links (active after the build_preview check completes)

New or renamed files

• https://docs-staging.datadoghq.com/lr/server-sdk-overview-docs/feature_flags/server/
• https://docs-staging.datadoghq.com/lr/server-sdk-overview-docs/feature_flags/server/go
• https://docs-staging.datadoghq.com/lr/server-sdk-overview-docs/feature_flags/server/java
• https://docs-staging.datadoghq.com/lr/server-sdk-overview-docs/feature_flags/server/nodejs
• https://docs-staging.datadoghq.com/lr/server-sdk-overview-docs/feature_flags/server/python
• https://docs-staging.datadoghq.com/lr/server-sdk-overview-docs/feature_flags/server/ruby

Renamed files

• https://docs-staging.datadoghq.com/lr/server-sdk-overview-docs/feature_flags/client/
• https://docs-staging.datadoghq.com/lr/server-sdk-overview-docs/feature_flags/client/android
• https://docs-staging.datadoghq.com/lr/server-sdk-overview-docs/feature_flags/client/ios

Modified Files

• https://docs-staging.datadoghq.com/lr/server-sdk-overview-docs/feature_flags/

[drichards-87]

Created DOCS-12928 for Docs Team review.