Recommend synthetic _source before disabling _source entirely

[ash9146]

Summary

The mapping-source-field.md reference page lists synthetic _source
as the first option to consider for reducing _source disk usage,
before disabling _source completely:

If disk usage is important to you, then consider the following options:

1. Using synthetic _source...
2. Disabling the _source field completely.

This production-guidance page (disk-usage.md) only covered the second,
more disruptive option under "Disable _source" - reindex, update, and
highlighting stop working once _source is disabled. Synthetic
_source avoids that tradeoff for most common field types by
reconstructing _source at retrieval time from doc_values instead of
storing it, so reindex and update continue to work normally.

What's added

A new "Reduce _source storage overhead" section, placed right before
the existing "Disable _source" section, that:

• Recommends synthetic _source as the option to consider first
• Notes it requires an Enterprise subscription
  (per elastic.co/subscriptions, synthetic _source has been an
  Enterprise feature since 8.17)
• Notes the reconstructed _source can differ slightly from the
  original document (field order, array handling), linking to the
  relevant reference section for details

This isn't presented as a strictly better replacement for the existing
"Disable _source" section - both stay, since synthetic _source
requires a subscription and has its own caveats, so fully disabling
_source remains a valid option for some readers.

Testing

• Docs build passes
• New section renders correctly between "Watch your shard size" and
  "Disable _source"