installation instructions for users?

### Describe the issue

You link to build instructions pages which apparently may not have those rather than describing how to use abseil in projects. I don't use abseil; I just use something that apparently now has abseil dependency (and doesn't say to compile it as an abseil project) so simply want to install abseil (not use it in a project) on my operating system system-wide, which I can't find out how to do.

### Steps to reproduce the problem

View 'build instructions'.

### What version of Abseil are you using?

0

### What operating system and version are you using?

Slackware GNU/Linux 15+current

### What compiler and version are you using?

gcc v13.2.0

### What build system are you using?

cmake v3.28.0

### Additional context

It seems (as usual with cmake) some actual undocumented build instructions may be as below. Please document these/etc. or change the possibly-misleading 'build instructions' webpage title.

```
mkdir build
cd build
cmake ..
make
make install
```

There also seems to be no (more professional) 'make uninstall', so I can't safely install abseil nor use the project that now has abseil dependency, rather than wait for an abseil package for my operating system.