Improvements to registering an extension (#2)

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | #9644 (symfony) and #3231 (symfony-docs)

1. The original text is good but the topic of that paragraph is not "extension conventions" but "registering an extension" (i.e. if you follow these conventions then register goes like so ..)

2. Cleaned up different terminology "Conventions" v "Standards" -> just conventions now

3. Move this part to a more logical place. First create then register, then modify for additional needs.
Changed due to comments in repo collab
typos, CS and PSR links fixed
[security] adding a tip about supported algorithm for 'hash' function
use a phpfunction link + vocabulary change (method -> function)
fix code block syntax
Merge pull request #3224 from peterkokot/patch-1

Typos
Merge pull request #3236 from flip111/patch-2

Improvements to registering an extension (#2)
[#3236] minor tweaks
Merge pull request #3325 from FlorianLB/supported_algorithms

[WIP] [security] adding a tip about supported algorithm for 'hash' function
Merge pull request #3345 from xabbuh/fix-build-errors

[Book][Validation] fix code block syntax
improve the serialization of custom user models

(cherry picked from commit 7b34eabe4b506c3e49e4dedbefee66c1612bb83d)
[#3307] Adding an additional note about deserialization issues

(cherry picked from commit 7325ec5044c0339275a9700e767d88707116f79d)
[#3307] Fix thanks to @xabbuh

(cherry picked from commit 38612bc190ab2f97f2fbd3f3bc18a591df153cbe)
Clarifying some details on serialize/unserialize and making it consistent with changes we recently made

(cherry picked from commit f285c5a572ee88e94a9314e152a5075243811d49)

Conflicts:
	cookbook/security/entity_provider.rst