Update documentation links for Allium, Covalent, and The Indexing Company data indexers

[BVK0515]

What changed? Why?
Changed : https://docs.base.org/base-chain/tools/data-indexers

Updated documentation links in the data indexers page to reflect current URL structures for:

• Allium: Updated destinations, Real-time API, and Batch-enriched documentation links to point to the correct documentation sections
• Covalent: Updated all Covalent links to point to the new Goldrush documentation site (goldrush.dev) instead of the legacy covalenthq.com URLs
• The Indexing Company: Updated contact link to point to the correct contact page

This ensures users can access the correct and up-to-date documentation for these data indexing platforms when using Base.

Notes to reviewers

• All link updates were verified against the provided documentation
• Only documentation links were updated; no content changes were made
• Covalent links now point to goldrush.dev which appears to be their new documentation platform
• The Increment link for Covalent was left unchanged as no updated URL was provided

How has it been tested?

• Verified all new URLs are valid and accessible
• Checked that link text accurately describes the destination
• Confirmed no broken links were introduced
• Manually tested that the updated links lead to the correct documentation sections

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/2	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 1 Sum 2