Corrected numerous typos in Markdown files.

[ker2xu]

Ragas is a great tool but the contents of the website can still be improved.

I corrected typos and unified some names (like langchain, Langchain -> LangChain) in Markdown documentation files. It seems that most of docs were written by a single author and lots of common typos were repeatly introduced. There are also some other problems about docs such as readability, categorization, clarification and more. I should have planned to polish them one by one but I may not have enough time to polish all of them, because I realized there are so many documentations after correcting typos for them. XD

This pull request only involves correcting typos and I recommend the contributor who is in charge of writing docs to have a look at the modifications to avoid introducing these typos in the future. You may also consider using LTex extension in VSCode extension market to recognize them (though it is not perfect). The matching ipynb files are not corrected since I do not figure out a method to identify those typos in ipynb files easily. You may consider to correct the ipynb files as well so you can generate Markdown files accordingly.