Add pattern filter to yml and check for valid links w/o opening files

[kaankabalak]

• This commit adds a Rules field to the .yml file, and a Filter field under it to exclude unwanted patterns from the markdown content fetched from Github.
• This commit also checks if the links are valid or not without opening the file. This change was done to improve performance by preventing the 'reveal' method from running for each document while the sidebar is being generated.
• When the first link in the list of Documents is broken, to avoid the wrong link to be highlighted on index.html, Gluegun now returns an error while generating the index page and asks the user to correct it.