docs: moved from ipnb files to md files

[jjmachan]

From now on documentation will be expected to written as .md files

we have a script to do the conversion make ipynb-to-md that will do the conversion.

• all generated md files should be have the _ prefix so that we know it's a converted files and generated md files are not expected to be edited manually.
• We can now use all the md formats available (but should have worked otherwise too)
  

you can visualize the diff here

• https://docs.ragas.io/en/latest/howtos/customizations/metrics/write_your_own_metric/ (Generated from ipynb)
• https://ragas--1482.org.readthedocs.build/en/1482/howtos/customizations/metrics/_write_your_own_metric/ (Generated from .md)

Do compare them side by side before merging

[shahules786]

There are places in which the these ipynb files were linked to which have to be changed. For example https://ragas--1482.org.readthedocs.build/en/1482/howtos/customizations/
Links under metrics does not work.

addressed these