nginx improvements

Page: https://ubuntu.com/server/docs/how-to/web-services/install-nginx/

for all sections:
- they should be in a dedicated subsection, not 3 separate nginx sections directly under webservers:

- code blocks:
- `$ ` should not be copied in the code blocks
- there should be a single copyable command for each block

### install section
- the bottom references are currenly broken:

```
Part 2: {ref}`How to configure nginx
Part 3: {ref}`How to use nginx modules
```

### configure section
- the example to modify the docroot should just mention to change `root` instead of a full config file where it's hard to see the change.
- tls setup should refer and explain let's encrypt setup primarily, the manual openssl commands and references are way less relevant in practice.
- the self-signing example should be part of the certificate article, not duplicated here in the nginx article

### modules section
- there should be a link at the top referring to the list of all available modules, i.e. https://nginx.org/en/docs/#:~:text=Modules%20reference

Open the linked nginx installation page and review its install, configure, and modules sections. Check the section structure, command blocks, broken references, docroot and TLS guidance, and module link against the issue; done means all listed documentation and organization problems are corrected and the page links work.

Written by the indexing model from the issue text.