[feature][search] Add env-gated Algolia DocSearch alongside local search

[dataroaring]

What

Enable Apache Doris docs to opt into Algolia DocSearch without removing the existing local search (@yang1666204/docusaurus-search-local).

Why

The Algolia Crawler needs a domain-verification token served from robots.txt, and we want to A/B Algolia against the current local search before fully cutting over rather than swapping it out in one step.

Changes

File	Change
static/robots.txt	Add Algolia Crawler domain-verification token
docusaurus.config.js	USE_ALGOLIA build flag; conditionally load the Algolia theme + themeConfig.algolia (read from env); expose customFields.useAlgolia
src/theme/SearchBar/index.js	Swizzle renders Algolia's bar or the local bar at runtime based on customFields.useAlgolia
package.json	Pin @docusaurus/theme-search-algolia@3.6.3 as a direct dependency

Design notes

• Additive, not a replacement. The local-search theme stays loaded in both modes so its @generated index module (which the proxiedGenerated swizzle depends on) keeps compiling.
• Modal-only DocSearch (searchPagePath: false) so it doesn't collide with the local /search route. contextualSearch: true keeps results scoped per version/locale.
• Default behavior is unchanged: with USE_ALGOLIA unset, the site builds and serves the existing local search exactly as before.

How to enable

USE_ALGOLIA=true \
ALGOLIA_APP_ID=xxxx \
ALGOLIA_SEARCH_API_KEY=xxxx \   # search-only public key
ALGOLIA_INDEX_NAME=doris \
yarn build

Testing

• Default (local-search) path: config passes node --check; logically untouched.
• ⚠️ The USE_ALGOLIA=true path has not been built end-to-end here (requires live Algolia credentials). Import path/default export for the Algolia SearchBar were verified, but a real USE_ALGOLIA=true build should be run before relying on it.

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