Convert markup generation to a function

[eschnett]

Using a function to generate markup ensures that its temporary variables (e.g. str) are not visible any more later on, avoiding compiler warnings.

[vtjnash]

Do you just want a let basename = "NEWS" block here (since the goal seems to be avoiding introducing unnecessary new global names)?

[inkydragon]

@vtjnash
I am going to add HISTORY.md to the documentation as well.
So defining a new function now looks acceptable.
I just need to add a new line of function calls: generate_markdown("HISTORY")

After some trying, I found that the generate_markdown function actually only applies to NEWS.md so it may not be necessary to define a separate function.

Using let is indeed a more proper way.

-function generate_markdown(basename)
+let basename = "NEWS"

...

end
-generate_markdown("NEWS")

Can we merge this pr now?
cc: @ViralBShah

[ViralBShah]

@vtjnash Can we merge?

[ViralBShah]

Actually this is pretty straightforwad. Merging.