Rjh/doc fixes for source install

What does this MR do?

Updates 2 key pieces of information in the documentation for source installations.

1. Adds NODE_OPTIONS="--max_old_space_size=4096" during asset creation to prevent out of memory errors from occurring as referenced in ##50937 (closed)
2. Adjusts location of backup command when changing versions as GitLab is required to be running for backups to occur as well as to also conform with how patch versions upgrade.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

No performance or testing changes needed.

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

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