[DOCS] Case file attachments

[lcawl]

Relates to #151595

The following documentation is be affected by this feature:

• Kibana:Open and manage cases Update screenshot and create new section about adding file attachments [DOCS] Case file attachments #156459
• Kibana release highlights [DOCS] Adds content for 8.8 #157343
• Case settings: https://github.com/elastic/cloud/pull/113684, [Cases] Adding cases settings docs #154672
• Case APIs N/A
• Observability: Open and manage cases Add text related to new file attachment feature [DOCS] Case file attachments observability-docs#2914
• Security: Open and manage cases: Update screenshots and create new section about adding file attachments [DOCS] Case file attachments security-docs#3236

Screenshots

[elasticmachine]

Pinging @elastic/kibana-docs (Team:Docs)

[elasticmachine]

Pinging @elastic/response-ops-cases (Feature:Cases)

[cnasikas]

@lcawl About the APIs regarding the file attachments should be considered internal and not supported.