[product documentation] adapt semantic_text usages to the new format

[pgayvallet]

semantic_text will face some breaking changes in 8.18 and 9.0 (see internal issue: https://github.com/elastic/dev/issues/2936)

We need to adapt the product documentation feature (artifact generation + installation) to support the new format.

Given the data changes, we will need to also generate 8.18 artifacts with the new data format. Note that given the 8.18 doc might not be online yet, so we could just generate the 8.17 artifacts again and label them as 8.18.

[elasticmachine]

Pinging @elastic/appex-ai-infra (Team:AI Infra)

[pgayvallet]

Enabling the new behavior can be done using an index setting:

"index.mapping.semantic_text.use_legacy_format": false

then exporting the embeddings can be done by requesting the _inference_fields metafield:

GET my-index-1/_search
{
  "query": {
    "match_all": {}
  },
  "fields": ["_inference_fields"]
}