DOC-2598: TINY-11459 Improve the context form placeholder sample code

[FarzadHayat]

Ticket: DOC-2598

Site: Release note entry
Site: Context Form

Changes:

• Improve the sample code in 7.6.0 release note entry
• Add placeholder option to Context Form specification table
• Add placeholder option to Context Form live demo

Pre-checks:

• Branch prefixed with feature/<version>/, hotfix/<version>/, staging/<version>/, or release/<version>/.
• [-] modules/ROOT/nav.adoc has been updated (if applicable).
• [-] Included a release note entry for any New product features.
• [-] If this is a minor release, updated productminorversion in antora.yml and added new supported versions entry in modules/ROOT/partials/misc/supported-versions.adoc.

Review:

• [-] Documentation Team Lead has reviewed