Simplify docs on enable Elasticsearch integration

What does this MR do?

1. Remove instructions for indexing small instances. It's not necessary since we already have instructions for enabling/indexing which works fine for small instances.
2. Remove the separation between "indexing via rake" or "via administration UI" considering that the via Rake option is only necessary/useful for very large instances
3. Remove the section describing the 1MiB file size limit because this is actually configurable now and the configuration option is already described at the top of this section as "Maximum file size indexed"
4. Reduce the extra link back to explaining backfilling data since this is part of the setup process and the big green button we have today which says "Index all projects" is much clearer than the previous user experience.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team