Refactor embeddings

What does this MR do and why?

This refactoring is part of the effort to add VertexAi embeddings support to GitLab documentation embeddings, see Draft: [PoC] Create embeddings with vertex ai (!129864 - closed)

This is some light code refactoring which moves a couple classes to a bit more relevant modules since our experimentation and also prepares some of the utility classes to be used for other purposes. Eg creating embeddings for bugger chunks of text by sending the text size limits as parameters.

Main changes:

• ee/lib/gitlab/llm/embeddings/utils/base_content_parser.rb
  • code was moved from ee/lib/gitlab/llm/content_parser.rb
  • class methods changed into instance methods
  • initializer added so that we can pass in different parameters for parsing and spliting content chunks for embedding
• ee/lib/gitlab/llm/embeddings/utils/docs_content_parser.rb
  • added DocsContentParser as a specific version of content parser for documentation.
  • DocsContentParser follows the same limits on min and max characters as original ContentParser
• Having the BaseContentParser should hopefully help out when we want to split issues content for embeddings for instance if we decide we want different limits for splitting content.

re https://gitlab.com/gitlab-org/gitlab/-/issues/420939

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