New read_first_record method and minor documentation fix

[lupemba]

I wanted to add historic IASI L1C formats but this was not needed. The current IASI L1C format is the same formats as the oldest files on the data store (2013). The format has been the same since 2010 so that is good enough.

I will just add the historic formats if someone request it.

Other changes made instead.

• Fix typo in documentation
• Remove stable documentation link in readme. We can added it again when we actually have a working link
• Add method to read_first_record for file path.