Add caching for generated API docs

[steve-calvert-glean]

This pull request introduces a caching mechanism to the docusaurus-plugin-openapi-docs plugin, which skips documentation generation if the source specification has not changed. The changes include updates to the plugin's configuration, implementation of the caching logic, and documentation updates. Below are the most important changes:

Plugin Enhancements

• Added a new cache option to the plugin configuration, defaulting to true, to enable skipping generation when the source specification remains unchanged. (packages/docusaurus-plugin-openapi-docs/src/options.ts, packages/docusaurus-plugin-openapi-docs/src/types.ts, [1] [2]
• Implemented caching logic using SHA-1 hashing to detect changes in the OpenAPI specification files. If the hash matches the previously stored value, generation is skipped, and a message is logged. (packages/docusaurus-plugin-openapi-docs/src/index.ts, [1] [2]
• Added cleanup functionality to remove the hash file during cleanup operations. (packages/docusaurus-plugin-openapi-docs/src/index.ts, packages/docusaurus-plugin-openapi-docs/src/index.tsR577-R590)

Documentation Updates

• Updated the plugin's README and demo documentation to include the new cache option and its default behavior. (packages/docusaurus-plugin-openapi-docs/README.md, demo/docs/intro.mdx, [1] [2] [3]

Codebase Improvements

• Introduced a utility function getSpecHashPath to manage the path for the hash file. (packages/docusaurus-plugin-openapi-docs/src/index.ts, packages/docusaurus-plugin-openapi-docs/src/index.tsR39-R44)
• Imported the crypto module for hashing functionality. (packages/docusaurus-plugin-openapi-docs/src/index.ts, packages/docusaurus-plugin-openapi-docs/src/index.tsR11)