docs: Fix broken links in implementation index and sharding doc

[imitater-dou]

Fixes three broken links in the docs:

• docs/implementation/README.md
  • drops the "Adding support for a new chain" entry — add-chain.md was removed in f593da5 (docs: Remove add-chain.md) but the index entry stayed
  • "Dump Format" pointed at ./dump.md, but the file was added as docs/dump.md (1bd0448), so the link resolves to docs/implementation/dump.md and 404s; changed to ../dump.md
• docs/sharding.md: the "here" link for ForeignServer::map_primary / map_metadata still points at store/postgres/src/connection_pool.rs. That file was moved under store/postgres/src/pool/ (a6dbe59) and ForeignServer was later split out into pool/foreign_server.rs (5d3d465), which is where both functions are now.

Docs-only, no code changes.

Verify

B=https://github.com/graphprotocol/graph-node/blob/master
# old targets -> 404
curl -s -o /dev/null -w '%{http_code}\n' $B/docs/implementation/add-chain.md
curl -s -o /dev/null -w '%{http_code}\n' $B/docs/implementation/dump.md
curl -s -o /dev/null -w '%{http_code}\n' $B/store/postgres/src/connection_pool.rs
# new targets -> 200
curl -s -o /dev/null -w '%{http_code}\n' $B/docs/dump.md
curl -s -o /dev/null -w '%{http_code}\n' $B/store/postgres/src/pool/foreign_server.rs

# the functions referenced in sharding.md
curl -s https://raw.githubusercontent.com/graphprotocol/graph-node/master/store/postgres/src/pool/foreign_server.rs \
  | grep -nE 'struct ForeignServer|fn map_primary|fn map_metadata'