Unify API Auth documentation

[pepopowitz]

Part of #3649.

Unify the content on our various API Authentication docs, so that each API Auth guide contains the following:

• Steps to generate a token, in either a SaaS (if available) or self-managed (if available) environment
  • Clear description of environment variables to be downloaded when creating a client
• An example auth call, in either a SaaS (if available) or self-managed (if available) environment
• An example usage of a generated token, in either a SaaS (if available) or self-managed (if available) environment
• A description of token expiration
• Consistent authentication requests between SaaS and self-managed, such that plugging the sample SaaS auth request into an OOTB self-managed environment with the correct variables, will work. (See Incorrect curl command to request bearer token #4092 (comment))

Tasks

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1. feat: unify API Auth docs (Admin, Camunda 8, and Web Modeler APIs) #4078
   3 of 3
   component:docs deploy theme:api-streamline +3
   
2. feat: rewrite Operate API auth guide #4138
   component:docs theme:api-streamline +2
   
3. feat: rewrite Tasklist API auth guide #4152
   component:docs theme:api-streamline +2
   
4. feat: rewrite Admin-SM API auth guide #4215
   component:docs theme:api-streamline +2
   
5. feat: rewrite Optimize API auth guide #4315
   component:docs deploy theme:api-streamline +3
   
6. Zeebe REST API (deprecated)
7. Apply Mark's feedback from feat: rewrite Optimize API auth guide #4315 to all rewritten auth guides