Begin using Maven Wrapper.
I set this up with:
```
mvn org.apache.maven.plugins:maven-wrapper-plugin:3.2.0:wrapper -Dtype=-script -Dmaven=3.9.5
```
It's not obvious to me whether we'll notice many differences among the 3 non-binary types (`source`, `script`, and `only-script`). I initially went with `only-script`, which is newest, given that it sounded theoretically like it would involve the fewest steps. But I got [an error](https://github.com/google/guava/actions/runs/6536542346/job/17748480689?pr=6783) from the Windows CI:
```
distributionUrl is not valid, must match *-bin.zip or maven-mvnd-*.zip, but found 'https://repo.maven.apache.org/maven2/org/apache/maven/apache-maven/3.9.5/apache-maven-3.9.5-bin.zip
'
```
I assume that this is a newline-related issue. That error message comes from the `only-script` shell script, and there is no such line in the `script` shell script, so I'm now trying `script`. (It's possible that we should be using `mvnw.cmd` under Windows, so maybe nothing will work until I change our CI to do that....
(We got a recommendation for the Maven Wrapper [from Ben Manes](https://groups.google.com/g/guava-discuss/c/e5UVvuM9CP8/m/w2H3CjZ7AQAJ).)
There does not yet appear to be a way to [make Dependabot automatically update the version of Maven we use](https://github.com/dependabot/dependabot-core/issues/485) (nor the version of the wrapper itself, though I looked less hard for that because it's less important). We've already not kept up with Maven upgrades very carefully—and we might each be using different Maven versions for all I know!—so this isn't necessarily a huge downgrade. Still, we don't want to fall far behind what's on our machines or GitHub CI, so I've at least set myself a calendar reminder to check back every 6 months.
RELNOTES=n/a
PiperOrigin-RevId: 573917287
Fix md files that were incorrectly marked executable.
Sync changes from migration to internal pages.
Primarily, fix up tons of Markdown stuff.
partially update for Github
Updated ContributorSetUp (markdown)
Fix wiki links to use proper format to work on GitHub.
Initial commit of Guava wiki contents.