lib/gis: markdown parser lowercase and extension fixes

[neteler]

This PR updates the Markdown support in the parser for the manual in terms of

• converting keywords to lowercase in manual pages so that e.g. dist.x86_64-pc-linux-gnu/docs/mkdocs/site/keywords.html#modis works
• replacing of .html with .md in certain cases

[neteler]

A still open issue is that in the page below the grass-XXX DB pages have a .md extension (should be .html):

dist.x86_64-pc-linux-gnu/docs/mkdocs/site/databaseintro.html -> Creating a database ---> linked pages in the section have the wrong file extension

All of them start with grass-, maybe that's triggering the issue?

Any ideas, @landam ?

[landam]

A still open issue is that in the page below the grass-XXX DB pages have a .md extension (should be .html):

dist.x86_64-pc-linux-gnu/docs/mkdocs/site/databaseintro.html -> Creating a database ---> linked pages in the section have the wrong file extension

All of them start with grass-, maybe that's triggering the issue?

Any ideas, @landam ?

@neteler Let's continue here: https://github.com/OSGeo/grass/pull/4742

[echoix]

I'm letting you two handle this #4740 and #4742 as I'm not sure in what order you'd want them in