docs(contribute): fix broken links, anchors and redirect targets around the Contribute tab

[palango]

Reviewing #2394 turned up links in and around the Contribute tab that were already broken before the rename. One opens the raw markdown export, four in-page anchors match no heading, one link is empty, and six old URLs redirect to the Contribute landing page even though the content they used to have lives elsewhere. This PR fixes those, makes the footer link relative, and retitles the landing page to match the tab. It builds on #2394 and targets that branch, so the diff shows only these fixes.

Refs #2261

Changes

Links in the moved pages:

• contribute/contributors/code-contributors.mdx:53: ./overview.md served the raw markdown export. It now links /contribute/contributors/overview.
• contribute/release-process/smart-contracts.mdx:303,342,390,407: the anchors now point at the headings that exist: #when-a-new-release-branch-is-cut, #release-management-in-git/github, #deploy-the-release-candidate and #verify-proposed-release-candidate.
• contribute/release-process/blockchain-client.mdx:28: the empty [docs.celo.org]() now links /learn/protocol.

Redirects in docs.json:

• The three celo-signal sources go to /learn/protocol/staking/voting, which has the Celo Signal signup. Their sibling celo-foundation-voting-policy redirects already point there.
• /general/ecosystem/builders and /what-is-celo/joining-celo/builders go to /contribute/builders, and /general/ecosystem/contributors goes to /contribute/contributors/code-contributors. git log --follow shows these old pages became those files.
• The footer "Contributors" href is /contribute instead of https://docs.celo.org/contribute/. Mintlify opens absolute footer links in a new tab, and from a preview build they lead to production.

Link targets:

• build/tools/oracles/run.mdx:11: "Celo core contract" now links the core contracts page, which lists the SortedOracles address on both networks, instead of the release process.
• build/tools/libraries-sdks/contractkit/index.mdx:25: removed the ContractKit link that led to the release policy page.
• operate/operators/faq.mdx:55: the Accordion title is plain text. Mintlify showed the markdown link there as text, and mint broken-links does not check attributes. The accordion before it already links the same page in its body.

Landing page: contribute/index.mdx is titled "Contribute to Celo" with sidebar label "Overview", in place of "Joining Celo".

Verification

On the branch head, with mint dev, the four new anchors match heading ids on the rendered page, and four of the six changed redirects (/validator/celo-signal and the three builders/contributors sources) return 307 to their new destinations. I did not curl the other two celo-signal sources; they have the same destination.

$ python3 scripts/check_redirects.py
No redirect problems found.
$ mint broken-links --check-redirects
success no broken links found
$ bash scripts/check-orphans.sh
No orphan pages found.

Once #2394 merges, I'll rebase onto main so the diff stays clean.

[GigaHierz]

Approving. Checked against the branch head:

• The four smart-contracts anchors match existing headings (When a new release branch is cut, Release management in Git/Github, Deploy the release candidate, Verify Proposed Release Candidate).
• /learn/protocol/staking/voting has the Celo Signal signup, so the three celo-signal redirects now land on the content they used to serve.
• /build/tools/contracts/core-contracts lists SortedOracles for both networks, which is what the oracle guide needs.
• CI is green, including the broken-links check.

One nit, not blocking: the PR says the landing page is retitled to match the tab, but the tab is "Contribute" and the title is "Contribute to Celo". Fine either way; it reads better as a page title.