docs(lanelet2_map_validator): add a new document how_to_contribute.md

[TaikiYamada4]

Description

This PR adds a document how_to_contribute.md to lanelet2_map_validator.

Related links

None

Tests performed

Not applicable

Notes for reviewers

None

Interface changes

None

Effects on system behavior

None

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[github-actions]

Documentation URL: https://autowarefoundation.github.io/autoware_tools/pr-170/
Modified URLs:

• https://autowarefoundation.github.io/autoware_tools/pr-170/map/autoware_lanelet2_map_validator/
• https://autowarefoundation.github.io/autoware_tools/pr-170/map/autoware_lanelet2_map_validator/how_to_contribute/