GitLab

Fix broken link.

PiperOrigin-RevId: 260548004

Preparation for including docs in round-trip with OSS, Manual merge, review and merge docs internal-OSS

ee3aa831 added a broken link
which results in 404. The broken part is reference to a
nonexistent file "CheatSheet.md" instead of "cheet_sheet.md".

This line directly contradicts the warning that google mock spits out on unused mock calls:
```
NOTE: You can safely ignore the above warning unless this call should not happen.  Do not suppress it by blindly adding an EXPECT_CALL() if you don't mean to enforce the call.  See https://github.com/google/googletest/blob/master/googlemock/docs/CookBook.md#knowing-when-to-expect for details.
```
One or the other should be changed, and I believe the advice in this file is incorrect.

Renaming doc files to make the file names more palatable and in preparation for including documentation in sync process

Removing make and automake. The only supported build systems are Bazel internally and CMake community supported

Added a missing newline

The Optional() matcher is otherwise undocumented except in the source.
This patch adds it to the cheat sheet for better visibility.

Removed "Documentation.md" not adding value and not consitent with internal docs

Rename AdvancedGuide.md to advanced.md and adjust the links. 
Part of documentation rationalization work