Add Base newcomer presentation docs

[codeforester]

Summary

• Add a text-first Base newcomer orientation deck under docs/presentations/.
• Add presentation README guidance that keeps markdown as canonical and documents optional PDF/PPTX export with Marp CLI.
• Link the deck from the documentation map and top-level README.

Validation

• env -u BASE_HOME ./bin/base-test baseline before edits: Python 369 passed, 1 skipped; BATS 394 passed.
• test -f docs/presentations/base-newcomer-orientation.md
• test -f docs/presentations/README.md
• rg -q "Base Newcomer Orientation" README.md docs/README.md docs/presentations/README.md docs/presentations/base-newcomer-orientation.md
• rg -q -- "--pdf" docs/presentations/README.md
• rg -q -- "--pptx" docs/presentations/README.md
• git diff --check
• git diff --cached --check
• git diff --check HEAD~1..HEAD

Demo Impact

None. Documentation and presentation packaging only.

Closes #546
Closes #547