Documentation: update and cleanup

[rnadyrshin]

What's new

In the doxygen documentation:

• Project name changed to «Flipper Developer Docs», active link added to the page header.
• Overview doc updated.
• Link to the Apps Catalog Contribution Guide added.
• Heatshrink file format doc and Number Input app example overview moved to the appropriate sections.
• Link to the app examples directory added to the “Apps examples” page.
• Word «Application» changed to «App» to align with the terminology used in other sources.
• Symbol «-» changed to «—» in accordance with the style guide.
• Symbols «->» changed to «→» in accordance with the style guide.
• Broken links fixed.
• Errors fixed.