Docs: use relative links for internal docs references

[HussainAther]

What?

This PR updates several documentation pages to use relative links when referencing other pages within the Hubs documentation, instead of absolute https://docs.hubsfoundation.org/... URLs.

The affected files are limited to:

• docs/dev-client-networking.md
• docs/dev-contributing.md
• docs/how-to-backup-your-Hubs-instance.md
• docs/set-up-SMTP-email-service.md

Why?

Docusaurus correctly resolves both absolute and relative links today.
This change is not required for correctness, but helps improve consistency and maintainability by:

• Keeping internal references environment-agnostic
• Avoiding hard-coding the public docs domain in source files
• Making future local or alternative deployments easier to reason about

This aligns with the longer-term goal of normalizing link formatting across hubs-docs.

Limitations

• This PR does not attempt to normalize all links across the repository.
• No style guide changes are introduced here.

Alternatives considered

• Leaving links as absolute URLs (current behavior)
• Waiting for a formal style guide before making any changes

This PR intentionally keeps scope small and focused to a few files.

Open questions

• Should relative links be preferred for all internal docs references going forward?
• If so, should this be codified in a documentation style guide?

[Exairnous]

Thank you for the PR. I have added this to the "review backlog from during sprints" item of the Communication & Documentation Team roadmap Google Doc to ensure this won't be forgotten about.