Move ES security-related documentation from the Stack Docs
to the Elasticsearch Reference Guide.

• Copy files from stack-docs repo to elasticsearch repo: [DOCS] Copies security source files from stack-docs #47521
• Fix pages that are missing anchors: [DOCS] Adds missing security anchors #47585 and Security anchor fixes stack-docs#576
• Create deleted pages for security content that is pulled from elasticsearch repo into Stack Overview and stop pulling it in (to break the dependencies between these files): Stages removal of security content stack-docs#574 and Security anchor fixes stack-docs#576
• Add security content into Elasticsearch book: [DOCS] Adds security content in the Elasticsearch Reference #47536
• Create deleted pages and remove security content from the table of contents in the Stack Overview: Removes security from Stack Overview stack-docs#575
• Move outstanding PRs from old repo to new repo
• Create redirects: Adds security redirects docs#1251, https://github.com/elastic/website-www.elastic.co/issues/4850
• Fix out-dated link: [DOCS] Fixes link to role mapping apm-server#2775, [DOCS] Fixes link to role mapping beats#13927, https://github.com/elastic/cloud/pull/41479, [DOCS] Cleans up links to security content #47610, [DOCS] Removes redundant CCS security kibana#47743, https://github.com/elastic/cloud/pull/41771, [DOCS] Fixes links to Stack Overview logstash#11239, [DOCS] Updates links to Stack Overview kibana#48582, [DOCS] Updates links to Stack Overview beats#14121, https://github.com/elastic/cloud/pull/42293

Improve content organization in new location:

• Amalgamate Active Directory realm pages: [DOCS] Merges duplicate pages for Active Directory realms #49205
• Amalgamate file realm pages: [DOCS] Merges duplicate pages for file realms #49200
• Amalgamate LDAP realm pages: [DOCS] Merges duplicate pages for LDAP realms #49203
• Amalgamate native realm pages: [DOCS] Merges duplicate pages for native realms #49198
• Amalgamate PKI realm pages: [DOCS] Merges duplicate pages for PKI realms #49206
• Amalgamate SAML realm pages: [DOCS] Merges duplicate pages for SAML realms #49209
• Amalgamate Kerberos realm pages: [DOCS] Merges duplicate pages for Kerberos realms #49207