Add API documentation

[aquatiko]

I have added the documentation from source docs and the original comments mentioned in the file.
Here I have added API docs only for documents.jl file. It's a bit long task to do for all the exported functions, I will be adding them gradually.

Closes: #146

[cchderrick]

I think examples that meant to run in repl can be formatted like this

```julia-repl
julia> sd = StringDocument(str)
    StringDocument{String}("To be or not to be...", TextAnalysis.DocumentMetadata(Languages.English(), "Untitled Document", "Unknown Author", "Unknown Time"))
```

[aquatiko]

Oh! thnx for the tip
I wasn't quite aware of this feature...

[Ayushk4]

Rohit, thank you so much for the newly add docstrings to hash.jl and dtm.jl.

I feel that some changes could be made for the newly added docs to dtm.jl, hash.jl . (Refer - https://docs.julialang.org/en/v1/manual/documentation/index.html )

Also, I feel that a separate argument list explanation shouldn't be necessary for each function. (See Point 4. in the link above).

[Ayushk4]

Adding DocStrings to the remaining files done in #150 . 🎉

[Ayushk4]

I have some changes to suggest based on the official julia documentation -

• Function signature should be written with a four spaced indent.
• The one line docstrings should be imperative form, followed by a blank line and then a 2nd paragraph.
• The first line should end with a fullstop.
• Some places arguments have been provided where it may not be necessary (Refer point 4 in docs).

[aquatiko]

@aviks This PR is complete from my side. Needs reviews from you..

[aviks]

Hi @aquatiko this looks good, but there is unfortunately a conflict. Could you resolve that please?

[aquatiko]

Done @aviks