[IMP] Sales: Big update to outdated discount and loyalty language

[fw-bot]

What this PR does and why it's needed

Updates the Discount and loyalty programs page in Sales documentation to fix outdated and inaccurate language flagged in reviewer feedback, including stale references to program templates and version-specific wording, and reorganizes the program form field list for clarity.

Changes

Discount and loyalty programs

• Added a page meta description for SEO
• Replaced the outdated description of program templates (which only appear before any program is created) with a list of the three default sample programs available out of the box: Code for 10% on orders, Buy 3 large cabinets, get one for free, and 10% Discount Coupons
• Reordered and clarified the program form fields list, moving Start Date, End Date, Limit Usage, and Available On earlier, and noting that available fields vary by program type and installed apps
• Renamed the sales/pricing_management/program-types anchor to sales/loyalty_discount/program-types and updated all internal :ref: references to match
• Generalized the note about the Loyalty Cards smart button on contact forms, removing outdated "Odoo 17" version-specific phrasing

Deleted files

• content/applications/sales/sales/products_prices/loyalty_discount/price-discount-loyalty.png — screenshot of the removed program-templates section, no longer referenced

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This 17.0 PR can be FWP up to master.

Forward-Port-Of: #19506
Forward-Port-Of: #19459

[robodoo]

[fw-bot]

This PR targets saas-18.3 and is part of the forward-port chain. Further PRs will be created up to master.

More info at https://github.com/odoo/odoo/wiki/Mergebot#forward-port