Run the main build on every push to main, not just root-file pushes

[mgravell]

Pushes to main mostly don't build, which makes it hard to read off the version to tag a
release with.

Cause

  push:
    branches: [main]
    paths:
    - '*'
    - '!/docs/*'

GitHub's * doesn't cross a /, so '*' matches only root-level files. A push touching
just Dapper/ or tests/ fails the filter and is skipped — silently, since a skipped workflow
isn't a failure.

commit	root-level files	main build
8becae8 (#2230)	4	✓ ran
2ca8261 (#2223)	1	✓ ran
6d48ef6 (#2228)	0	✗ skipped
41d76c7 (#2225)	0	✗ skipped

The '!/docs/*' line never worked either — path filters are repo-relative and take no leading
slash, so it matched nothing. The docs exclusion it was written for was happening by accident,
via the '*' that already excluded every subdirectory.

Fix

    paths-ignore:
    - 'docs/**'

Which is what was meant: build everything except a docs-only change.

Why it matters now

release.yml reads the version to tag from a green main run's summary. A main build that never
runs means no version — which is how this surfaced. PR builds were unaffected throughout (the
pull_request trigger has no path filter), so CI looked healthy.

Merge order

Worth merging before #2237: that one only touches Dapper.StrongName/ and
Dapper.EntityFramework.StrongName/, so under the current filter it would merge without
producing a main run or a version. This PR's own merge commit will build, since push events
evaluate the workflow file at the pushed commit.