ci: make sure changes in lance-namespace-impls doc is auto-updated

[jackye1995]

In previous change, we manually added each supported catalog integration, but that means every time we add a new one we still need to make a PR to lance, and that is inconvenient. This PR makes it simpler by having the nav .pages in the lance-namespace-impls repo and just add the template in the end, so we only need to update that repo for new catalog implementation supports.

[github-actions]

PR Review

Summary: This PR simplifies the docs-deploy workflow by copying all markdown files via wildcard and using the .pages file from lance-namespace-impls instead of manually maintaining the list in this repo.

P1: Potential Issue with template.md

The old workflow referenced template.md in the generated .pages file, but it was never copied from anywhere. It appears to be expected to exist in the integrations folder (likely created by the lance-namespace repo step). The new approach appends this entry to the copied .pages file.

Questions to verify:

1. Does template.md exist in lance-namespace/docs/src/integrations/ (copied earlier by the Copy lance-namespace docs step)?
2. If not, does lance-namespace-impls/docs/src/.pages already include the template entry, making the echo redundant or causing a duplicate entry?

If template.md does not exist anywhere, the docs build may fail with a missing file error.

Minor: Wildcard copy behavior

The wildcard copy will copy all markdown files, including any future additions. This is the intended behavior, but ensure there are no markdown files in that directory that should not be copied (e.g., a README.md that is only for the source repo). This is low risk if the source repo is maintained with this in mind.