docs(contributing): document pr base branch and mdx page format

[youssefea]

What changed? Why?

• Adds an "Opening a Pull Request" section to docs/contribution-guidelines.md. It says to open PRs against master, to create new pages as .mdx files because the Docs Style / Conformance check and orphan detection only cover .mdx files, and that snippets and .mintignore files are exempt from page-level lint rules. It also notes what node scripts/lint-mdx.js checks when you run it with no arguments.
• Updates the README "Submission process" step to name the master base branch and the .mdx requirement, with a link to the new section.

Closes knowledge-base gap gap-20260930-new-docs-org-pages-pr-branch: "For a new docs.base.org page, which branch in base/docs should a contributor base their PR on, and must new pages be MDX (not Markdown) for the Docs Style / Conformance lint to apply?" (gaps log, DEVREL-859).

Notes to reviewers

Primary sources for each fact:

• Default branch is master: gh api repos/base/docs --jq .default_branch.
• Style check covers only changed docs/**/*.mdx: .github/workflows/docs-style-conformance.yml ("Collect changed documentation pages" step, grep -E '^docs/.*\.mdx$').
• Linter skips non-.mdx paths, docs/snippets/, and .mintignore entries: isLintablePage and NON_PAGE_PREFIXES in scripts/lint-mdx.js.
• Orphan detection only scans .mdx: scripts/validate-docs-structure.js (orphan detection section).
• No-argument linter run compares against master and includes uncommitted changes: getChangedFiles in scripts/lint-mdx.js.

The guidance says .mdx is required for lint coverage. It does not say .md pages fail to render.

This replaces the contributor-guidelines/README part of #2087. The security page description fixes in #2087 are not included here.

How has it been tested?

• node scripts/lint-mdx.js --files-from=<changed list>: passes (no .mdx pages changed).
• node scripts/validate-docs-structure.js: fails with 2 redirect errors for /upgrades/denim/eip-8130. The same errors appear on unmodified master, so this change didn't cause them.
• Checked that the external link https://github.com/base/docs returns 200 and that the README anchor #opening-a-pull-request matches the new heading.

Screenshots

N/A (text-only change)

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[cb-heimdall]

🟡 Heimdall Review Status

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[linear]

DEVREL-859