Ensure Overview points to index.html and remove /overview from URLs

[otisg]

What file does http://sematext.com/docs/ open? I'm guessing index.html What does http://sematext.com/docs/logs/ open? Ooops, nothing. Why? Because there is no index.html

This is just one example of this sort of problem. There may be other such cases under /docs

We should:

• remove /overview from the URL. Instead of opening http://sematext.com/docs/logs/overview/ when one clicks on Log Management > Overview, we should open http://sematext.com/docs/logs/
• keep "Overview" as the name in the navigation on the left side

CC @megastef

[megastef]

Done for logs, monitoring, sematext-docker-agent and logagent. See https://github.com/sematext/docs/commit/3090d54bf8ec276188c0facc4c7056d4c660bce6 https://github.com/sematext/docs/commit/ca499620666edaf2823bf6c4e6bc608824d6923c https://github.com/sematext/docs/commit/17b46ed0701ad103e4a01bd2027c44d9c26b2c02

[nmtien]

I fixed the PDF links for this changes.